Start by preheating your oven to 350 degrees F. A jelly roll pan with a 10- by 15-inch size should be greased and floured.
Sift the baking powder, and flour together in a medium bowl and leave aside.
In a large bowl, use an electric mixer to beat the eggs, vanilla, and sugar together until the mixture becomes light and lemon-colored. Then, gradually beat in the flour mixture until it is well combined.
In a microwave-safe dish, place the milk and margarine together, then microwave the mixture until it begins to bubble, which should take around 2 minutes.
Once the milk and margarine mixture begins to bubble, pour it into the bowl with the batter and stir until well combined. Then pour the batter into the prepared jelly roll pan.
Bake the cake in the preheated oven for 20 to 25 minutes, check the cake for doneness by inserting a toothpick into the center. If it comes out clean, the cake is done. Let the cake cool completely on a wire rack.
After the cake has cooled completely on a wire rack, use a spoon to drop dollops of peanut butter all over the surface of the cake. Spread the peanut butter evenly on the edges of the cake. Then, refrigerate the cake for about 1 hour to allow the peanut butter to set.
Next, melt the chocolate in a microwave-safe dish and pour it over the peanut butter layer. Spread the chocolate evenly to the edges of the cake. Then, refrigerate the cake to allow the chocolate to set and harden.