Bake your favorite chocolate (or other flavor) cake and let cool.

In a small saucepan, whisk flour into milk and heat, stirring constantly until it thickens. You want it to be very thick -- thicker than a cake mix.

Remove from heat and let cool to room temperature. (If you're in a hurry, you can put the saucepan in a bowl with ice in the sink. It must be completely cool before you do the next step.)

Stir in vanilla.

While the mixture is cooling, cream the butter and sugar together (on very high) until light and fluffy. You don't want any graininess left.

Then add the completely cooled milk/flour/vanilla mixture and beat the living daylights out of it. If it looks separated, you haven't beaten it enough. Beat it until it all combines and resembles whipped cream, with stiff peaks.