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Once the sugar syrup has reached the right temperature, turn off the heat. Slowly add the cream and butter mixture to the syrup, whisking gently to fully incorporate it. After you’ve finished adding all of the cream and butter to the pan, stop whisking. When you mix in the cream and butter, the sugar syrup will triple in size as it bubbles up. Don’t be alarmed. Once the cream mixture is fully incorporated into the sugar syrup, turn the heat under the pan back up to medium or medium-high, and bring the caramel mixture to a boil. Don’t stir the caramel as it boils. Remove the pan from the heat when the mixture reaches 245 to 250 degrees Fahrenheit (118 to 121 degrees Celsius). The caramel mixture will start out a golden color, but gradually deepen to a reddish-brown caramel shade as it cooks. Right after you remove the caramel from the heat, whisk in ½ teaspoon (2.5 ml) vanilla extract into the mixture. Work quickly so the mixture doesn’t get too thick before you fully incorporate the vanilla.

Summary:
Combine the butter and milk mixture with the sugar syrup. Bring the caramel mixture to a boil. Stir in the the vanilla.