Preheat oven to 350°Fahrenheit. Line a 10×15″ jelly roll pan with foil or parchment paper. Spray with cooking spray.
In a large mixing bowl beat the eggs, sugar and vanilla until light and fluffy.
Sift the flour together with the baking powder and add to the egg mixture a little at a time while stirring on low.
Heat the milk together with the butter until the butter has melted and fold into the flour mixture.
Pour the batter onto the prepared baking sheet and bake for 20-25 minutes, until a toothpick comes out clean.
Spread the peanut butter gently over the warm cake. This will help the peanut butter to spread evenly.
Refrigerate for 30 minutes to cool.
In a medium sized microwave safe bowl, heat the chocolate and the shortening in the microwave in 30 second increments, stirring between each interval until melted. As the chocolate starts to melt, lower the time to 15-20 second intervals.
Pour the melted chocolate evenly over the refrigerated peanut butter and spread quickly and gently with a spatula. If you work with this step too long you’ll end up combining the two layers.