Preheat oven to 180 degrees C. Grease and flour a 20x40cm tin. Sift together the flour and baking powder; set aside.
In a large bowl, combine eggs, sugar and vanilla. Beat on high speed of an electric mixer until light and lemon-coloured. Beat in the flour mixture. Microwave milk and margarine about 2 minutes, or until it begins to bubble. Stir into batter.
Pour batter into tin. Bake in the preheated oven for 20 to 25 minutes, or until a toothpick inserted into the centre of the cake comes out clean.
When cake is cool, plop peanut butter all over and spread evenly to the edges. Refrigerate to set, about 1 hour.
In a microwave safe dish, melt chocolate and spread evenly over peanut butter layer. Refrigerate to allow chocolate to harden.