Preheat the oven to 350F. Grease and line a 9x13-inch baking tin with baking paper.
In a medium bowl, whisk together the flour, baking powder and salt.
In a large bowl, or the bowl of a stand mixer with paddle attachment, beat together both sugars, peanut butter, oil and vanilla until fully combined. Scrape down the sides.
Add the eggs, one at a time, beating well between each and scraping sides of bowl as necessary.
Add 1/3 of the flour mixture and fold through with spatula. Follow with half the milk and mix through gently, then repeat the process - flour, milk, flour.
Spread the batter into the prepared tin and bake for 18-20 minutes until a toothpick inserted comes out with just a crumb or two but not wet batter.
Make the peanut butter frosting:
Beat together the butter, peanut butter and 1/4 cup of powdered sugar until smooth and lightened.
With the mixer on low, add the sugar quarter of a cup at a time and continue to beat until fully combined.
Add the vanilla and salt, then beat on high for 2 minutes until fluffy and creamy.
Spread over the cooled cake then scatter with chocolate shavings.
It's always best to weigh flour using a kitchen scale to give the best results. If you use cups, make sure to fluff up the flour first, then spoon and level.This cake will keep well for 3-4 days when covered.
Peanut Butter Sheet Cake
Amount Per Serving (1 bar)
Calories 258Calories from Fat 135
% Daily Value*
Saturated Fat 6g38%
Trans Fat 1g
Polyunsaturated Fat 3g
Monounsaturated Fat 6g
Vitamin A 209IU4%
* Percent Daily Values are based on a 2000 calorie diet.