Start by preparing the ermine frosting:
In a medium saucepan, whisk the flour and milk over medium-high heat until it bubbles and thickens- it will resemble thick, pasty pudding. Stir in the vanilla, scrape it into a bowl, and set aside to cool with a piece of plastic wrap pushed down on the surface to prevent it from getting a skin. Chill in refrigerator for 2 hours.

Make the yellow boxed cake mix except
- add one additional egg
- use melted butter instead of oil
- use milk instead of water

Note: We really like to spread a little shortening on the inside of the cake pan(s), add a small spoonful of AP flour, cover with cling wrap, and shake the shit out of it until it's fully coated, then carefully dump out the excess. Perfect release every time!

Cool the cake for at least 1 hour.

Bring the ermine goo to room temperature for 30 minutes. Prepare the stand mixer, and beat the butter and sugar for a few minutes in a stand mixer until pale and fluffy. Add the cooled milk mixture in spoonfuls, whipping for a good 5 minutes or so, until the mixture resembles whipped cream. if the sugar is still a bit grainy, keep whipping until it isn't. Add the cocoa powder if you want chocolate frosting and whip until well incorporated.