For the cake, prepare yellow cake according to package directions (for 2 9-inch layer cake pans). Cool completely.
In a small saucepan, combine the flour with milk, whisking until lumps removed. Turn on medium heat and continue to whisk until mixture becomes thick, like a pudding.
Remove from heat and stir in vanilla. Cool completely.
In a mixing bowl, combine butter and sugar, beat until light and fluffy (about 4 minutes). Slowly add in the COOLED milk mixture, beating until combined. It may curdle slightly in the beginning, that's okay, keep beating until it becomes the texture of whipped cream!
Place first layer of cooled cake on a cake plate. Top it with ALL of the cream filling. Add the second layer of cake on top.
Sprinkle generously with powdered sugar. Slice and enjoy!
Notes
Bake the yellow cake in two 9-inch layer pans. Remember to line the bottom of your pans with a layer of parchment paper and baking spray. Or use my homemade cake release instead!
I use Wilton Bake Even strips for the cake layers. These little strips ensure the layers stay even on both sides while baking, creating a perfect appearance and great taste.
Let the milk and flour mixture cool completely before adding it to the butter and cream. Keep beating until it becomes the texture of fluffy whipped cream.
Use a sifter or confectioner’s sugar duster to add the powdered sugar to the top of the cake for a smooth appearance and consistency.