Preheat oven to 350. Line 2 (12 cup) muffin pans with paper liners. In a large bowl, beat peanut butter, brown sugar, sugar, and peanut butter at medium speed until fluffy. Add egg yolks, one at a time, beating well after each addition. In a medium bowl, combine flour, baking powder and salt. Gradually add to butter mixture alternating with milk, beginning and ending with flour mixture, beating just until combined after each addition. In a medium bowl, beat egg whites at high speed with a mixture until stiff peaks form, gently fold beaten egg whites into butter mixture. Spoon batter into prepared muffin cups, filling two-thirds full. Place 1 chocolate covered peanut butter cup in center of each muffin cup, pressing down until edges of peanut butter cup are even with cupcake batter. Bake for 15-20 min or until edges of cupcakes are golden brown, let cool in pans for 10 minutes. Remove from pans, and cool completely on wire racks. Spread, or pipe, creamy peanut butter frosting over cupcakes.
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