Aw, you youngsters, spoiled with your Nestle Quick... :)
To mix cocoa powder with a liquid (or really, to mix any powder with a liquid - salt and granulated sugar aren't powders), you need to make a slurry by mixing a small part of the liquid into all of the powder.
Then you can dilute the slurry with the rest of the liquid.
Note that if you're using sugar for sweetening, it helps to add it to the cocoa powder before you add any liquid, because the sugar helps the dissolving.
However, since stevia is so much sweeter than sugar, you use too little of it to make any difference to the dissolving process.
Thus, you can go ahead and add it afterward, so you can adjust the sweetness better.
cooking.stackexchange.com/users/2569/Marti
You could do it the same way that you make chocolate syrup:
Heat some water to boiling, dissovle the cocoa and sweetener and reduce down till it reaches the consistancy that you desire.
This will result in a syrup that will mix in to cold or hot drinks with no problem and can also be used to top ice cream and or other desserts.
cooking.stackexchange.com/users/446/sarge_smith
Did you try mixing the cocoa and stevia together then adding the milk?
That should help.
Also, you could make a paste with the cocoa and a little bit of milk, then adding a bit more milk to make a concentrated chocolate milk base.
At this point, you will be able to stir in the rest of the milk with ease.
Cocoa powder also mixes better in warm liquids.
Try making some hot cocoa...
Way better than the prepackaged powder stuff they sell at the market.
cooking.stackexchange.com/users/2239/Juju
If you are a regular cocoa drinker
another solution is to make a large quantity of cocoa mix in advance, using the method described above by Marti, then store in the fridge to use as required.
Blend the cocoa powder
/sweetener with water rather than milk as it will stay fresh in the fridge much longer.
A further suggestion is to pour the cocoa mix into ice-cube trays and freeze to store indefinitely.
Just put the number of required cubes in a cup and pour hot milk over, or use cold milk and reheat in a microwave.
Very quick, and no mess to clean up afterwards.
cooking.stackexchange.com/users/12772/user12772
The method I use for my own home-made milk-powder-free cocoa powder involves this process: Pour about 1/3rd of a mug of milk.
Microwave for 1 minute.
Stir in powder.
With a round whisk that fits inside the mug, I roll it between my hands to get good mixing action.
Microwave
another 10-20 seconds.
This is what really gets the chocolate melting.
Repeat step 4 Fill mug to desired level Microwave another minute.
Repeat step 4. Looks like a lot of work, but it really gets the job done.
No lumps, thanks to sufficient heat and a lot of manual labor.
Also?
It can be done in one mug.
cooking.stackexchange.com/users/798/sysadmin1138
I mix cocoa and Stevia in a small amount of hot water to form a paste.
Then I pour that into unsweetened almond milk (Almond Breeze) which I shake in a bottle 500ml.
cooking.stackexchange.com/users/10880/Sophia
Use a blender, as someone already mentioned.
A different trick is crushing the lumps on the side of the glass or cup then stirring, then setting it aside to kinda like mix by itself.
After about 1/2 hour try stirring it again.
Then you'll see that it does dissolve more readily.
For even more smoother consistency repeat the "crushing the lumps" steps.
How do I know it works?
At this very moment I am enjoying a large plastic glass (probably 10 ounces) of unsweetened Hershey's cocoa (the kind they use for cooking).
I sweetened it with corn syrup, and is it ever delicious!
BTW, I use soy milk.
cooking.stackexchange.com/users/18068/the humble guy
You won't believe this but quite simply use a blender to blend them together.
i tried dropping the powder in first and no way.
it takes way too much milk to dissolve the powder on the bottom.
try milk first then the powder mixture you made.
one point, the blender makes a nice frothy top for the drink.
I used whole milk, homemade nestle quick mix from hershey cocoa.
cooking.stackexchange.com/users/23492/robert
Valrhona Cocoa powder is quite expensive, but it has a much nicer flavor than Hershey and Nestle (much less acrid after-taste
so you can go darker without as much acridness) and it will dissipate nicely into even a cup of cold milk with simple stirring.
One is just trying to dissipate it without clumping as cocoa powder doesn't ever actually dissolve in water, which is why pretty much no matter what you do, cocoa powder will eventually settle to the bottom.
cooking.stackexchange.com/users/5119/John Robertson
I add cocoa powder and sweetener to a mug, then add skim milk.
Microwave for a minute or until almost boiling, and it should be completely dissolved.
Since you're trying to make chocolate milk not hot cocoa, just chill it afterwards.
cooking.stackexchange.com/users/32983/Liya
I wanted to make a Stevia-sweetened chocolate milk so I tried mixing some unsweetened cocoa powder with the milk and stevia, but I found that the cocoa powder doesn't mix with the milk at all. It tends to float on top, and even with a lot of stirring and mixing back and forth between two glasses it was a lumpy mess.  Any techniques or natural additives I can use to help the powder dissolve more like Nestle Quick and the like?
Dissolving cocoa powder in milk
cooking.stackexchange.com
<chocolate><milk>
cooking.stackexchange.com/questions/8274
cooking.stackexchange.com/users/1962/G__
Mix the cocoa powder and Stevia together, add a little of the liquid and make into a slurry before adding the rest of the liquid. You can also mix the powder in with a blender. Make a paste with the cocoa and liquid first. You can also use a blender.
Mix up a paste with the powder and a small amount of warmed liquid. Use a blender. Mix the cocoa and stevia together first.