In a large bowl, shift together the gluten free flour blend, cocoa powder, baking powder, baking soda, and a pinch of salt. Set aside.
In another large bowl, beat the softened butter along with the sugar for 2 to 3 minutes, until creamy. Add the egg, milk, vanilla extract and beat for another minute.
Add the entire bottle of red food color along with the vinegar. Beat for another few seconds.
Slowly add the flour mix, about a cup at a time, beating just until mixed. Do not over mix.
Pour the prepared batter into a well-greased and floured 6 or 7 inch round pan, or a 6-cup mini Bundt pan. Tap on counter to release any air bubbles and to redistribute the batter evenly.
Add 1½ cups of water in the Instant Pot insert. Place the cake pan on a trivet and gently lower it in the Instant Pot.
Select the “Pressure Cook” mode and adjust the time with the “- / +” to 30 minutes. Allow a 10 minute natural pressure release. Remove cake from IP and let set for 10-15 minutes. Run a knife around the pan and then invert onto a cake plate.
Beat the softened butter with the confectioner’s sugar until thick, smooth and creamy. Add the vanilla extract and the milk and beat for another few minutes. The glaze should be thick yet pourable. If it is too thick, add a teaspoon or two of milk. If it is too runny, add ¼ cup of sugar and beat for 30 seconds. Drizzle the glaze over the cooled red velvet cake.