Preheat oven to 350°F. Prepare cake pans: grease pans, lay a circle of parchment on the bottom, grease the parchment, and then flour pans. If making cupcakes just line tin with cupcake papers.
In a large bowl cream together peanut butter, butter, and sugar until fluffy. Beat in eggs. In a small bowl mix flour and cake enhancer.
Alternate stirring in flour and milk into butter mixture, starting and ending with flour, until batter is smooth.
Pour into prepared pans and bake for 25-35 minutes, depending on size of pan, until a toothpick inserted in the center comes out clean. Let cool on a rack for five minutes, then remove from pans and finish cooling. (If you're in a hurry, tightly wrap the cakes in plastic wrap, then place in the freezer. The resulting steam will help keep them moist, and the freezer jaunt will make frosting easier.)
To make the frosting, beat together butter, powdered sugar, and cocoa powder until well-mixed and fluffy. Beat in chocolate extract, and the milk a little bit at a time, until you get desired consistency. (You might not use all the milk, or you might need more.) Frost cakes, and store any extra in an airtight container in the fridge.
Notes
Makes 6-inch cake and 12 cupcakes (or 1 9-inch cake, or 24 cupcakes)