Preheat the oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a large mixing bowl, combine flour, sugar, cocoa powder, baking powder, and salt.
In a separate bowl, whisk together melted butter, eggs, vanilla extract, and milk until well combined.
Pour the wet mixture into the bowl of dry ingredients and stir until just combined.
Spread half of the batter in the prepared baking pan, then dollop the creamy peanut butter over the top.
Pour the remaining batter over the peanut butter layer and spread evenly.
Bake in the preheated oven for 25-30 minutes until the surface looks set but the center is still soft.
Let the cake cool in the pan for at least 15 minutes before serving, optionally dusting with confectioners' sugar.