2cups + 2 tbspall-purpose flour,fluffed, spooned and leveled
1 ½tspbaking powder
½tspcoarse Kosher salt(if using table salt, use half the amount)
Frosting
½cupunsalted butter
¼cupcreamy peanut butter
1cuppowdered sugar
½tspvanilla extract
⅛tspcoarse Kosher salt
flaky finishing salt(if using table salt, use half the amount)
Instructions
Cake
Preheat the oven to 350 F, and position a rack in the center of the oven. Lightly spray an 8 or 9 inch square baking pan with non-stick baking spray, or line with parchment paper.
In a saucepan over medium heat, melt the butter. Add the peanut butter and milk, and stir until smooth. Remove from the heat.
In a bowl, whisk together the warm peanut butter mixture with the granulated sugar and light brown sugar. Whisk in the eggs and vanilla.
Separately, combine the flour with the baking powder and salt. Add the dry ingredients to the wet ingredients and stir together just until combined. The batter will be thick.
Spread the batter into the prepared pan, and bake for about 30-35 minutes (for an 8 inch pan) or 25-30 minutes (for a 9 inch pan), until a toothpick or cake tester inserted in the center comes out clean, or with moist crumbs clinging to it.
Let the cake cool in the pan for 1 hour, then add the icing while the cake is warm.
Icing
In a saucepan over medium heat, melt the butter. Stir in the peanut butter until smooth. Stir in the powdered sugar, vanilla and salt. Stirring constantly with a rubber spatula, bring the frosting to a simmer, and remove from the heat when it begins to bubble.
Immediately, pour the frosting over the cake. Let sit for several hours, until the frosting is firm and set. Sprinkle the top with flaky finishing salt, then cut into squares.