Preheat oven to 350 degrees F. Sift together flour, baking powder, and salt in a medium-sized bowl. Set aside.
Cream the butter using an electric mixer at medium speed until light and fluffy. Add the peanut butter and continue to blend until smooth. Add all the brown sugar and beat until creamy.
Lightly beat the egg with the vanilla in small bowl. Slowly add this to the peanut butter mixture, stopping to scrape down the bowl to ensure ensure mixing. Make sure the egg is room temperature as not not curdle or break the peanut butter mixture.
Combine the whole milk and heavy cream (half and half blend). Add the flour mixture in three batches to the peanut mixture. Alternate with the half and half blend and mix well after each addition.
Use a large ice cream scoop to evenly divide the batter among the lined cupcake pan.
Bake for 20 minutes, until light golden. Be sure to test the center with a toothpick and make sure it comes out clean. Remove cupcakes from pan and let cool on rack before frosting.