Where did I go wrong in proving $\mathbb E[X^{2n}] = \prod_{1 \leq k \leq 2n, k \operatorname{odd}}k$ Let $X$ ~ $\mathcal{N}(0,1)$. Show that: $\displaystyle \mathbb E[X^{2n}] = \prod_{1 \leq k \leq 2n, k \operatorname{odd}}k$

Idea:
$$\mathbb E[X^{2n}]=\frac{1}{\sqrt{2\pi}}\int_{\mathbb R}x^{2n}e^{-\frac{x^2}{2}}dx=\frac{2}{\sqrt{2\pi}}\int_{0}^{\infty}x^{2n}e^{-\frac{x^2}{2}}dx~,$$ then set $t=x^2 / 2 \Rightarrow dt=xdx$ and
$$\frac{2}{\sqrt{2\pi}}\int_{0}^{\infty}x^{2n}e^{-\frac{x^2}{2}}dx=\frac{2^{n+1}}{\sqrt{\pi}}\int_{0}^{\infty}t^{n-\frac{1}{2}}e^{-t}dt=\frac{2^{n+1}}{\sqrt{\pi}}\Gamma(n+\frac{1}{2})$$
I know that in order to get the desired result, I need to show that:
$$\frac{2}{\sqrt{2\pi}}\int_{0}^{\infty}x^{2n}e^{-\frac{x^2}{2}}dx=\frac{2}{\sqrt{\pi}}\Gamma(n+\frac{1}{2})$$
But I am far off it, where did I go wrong?

You were actually off only by a factor of 2. With your choice of change of variable,
\begin{align}
\frac2{\sqrt{2\pi}}\int_{0}^{\infty}x^{2n}e^{-\frac{x^2}2 }dx &= \frac2{\sqrt{2\pi}}\int_{0}^{\infty}x^{2(n - \frac12)} e^{-\frac{x^2}2 } \cdot xdx \\
&=\frac{ 2^{1/2} }{\sqrt{\pi}}\int_{0}^{\infty}(2t)^{n - \frac12} e^{-t^2 } \cdot dt \\
&= \frac{2^n}{\sqrt{\pi}}\int_{0}^{\infty}t^{n-\frac12}e^{-t}dt=\frac{2^n}{\sqrt{\pi}}\Gamma\Bigl( n + \frac12 \Bigr) 
\end{align}
Invoking the definition of Gamma function that $\Gamma(x) = (x-1)\Gamma(x-1)$, for $n \in \mathbb{N}$ we have
\begin{align}
&\phantom{ {}={} }\frac{2^n}{\sqrt{\pi}}\Gamma\Bigl( n + \frac12 \Bigr) \\
&= \frac{ 2^{n-1} }{\sqrt{\pi}} \cdot 2\cdot \Bigl( n - \frac12 \Bigr)\cdot \Gamma\Bigl( n - \frac12 \Bigr) \\
&= \frac{ 2^{n-2} }{\sqrt{\pi}} \cdot (2n - 1) \cdot 2\Bigl( n - \frac32 \Bigr) \cdot \Gamma\Bigl( n - \frac32 \Bigr) \\
& \hspace{36pt}\vdots \\
&=\frac1{\sqrt{\pi}} \cdot (2n-1) \hspace{-10pt} \underbrace{(2n-3)}_{\text{from}~n-\frac12-(1)~\text{so 2nd}}  \hspace{-24pt}  \overbrace{(2n-5)}^{\text{from}~n-\frac12-(2)~\text{so 3rd}} \hspace{-12pt} \ldots 5\cdot 3\cdot \hspace{-24pt}  \underbrace{\Bigl(2 \cdot \frac12 \Bigr)}_{\text{from}~n-\frac12-(n-1)~\text{so $n$ th}} \hspace{-24pt}  \cdot\Gamma\Bigl( \frac12 \Bigr) \\
&= (2n-1)(2n-3)(2n-5)\cdots 5\cdot 3 \cdot 1
\end{align}
as desired, where it is well-known that $\Gamma(1/2) = \sqrt{\pi}$.
