Preheat oven to 350℉. Grease a 9×13 pan with unsalted butter; set aside.
In a standmixer fitted with the whisk attachment add the eggs, and sugar. Beat until thick, white, and ribbon light; about 7 minutes. When you remove the whisk the mixture should have a stretch to it like a long ribbon.
Add in the vanilla extract and butter and whip for an additional 2 minutes.
Fold in the blueberries and remove the bowl from the mixer.
Add the thickened cake batter to the greased pan and spread evenly using a rubber spatula.
Place on the center oven rack and bake for 40-50 minutes or until the top is flaky, and golden brown. A tester inserted into the center of the cake should come out clean.
Allow the cake to cool for 30 minutes before cutting into squares. Enjoy!
