Make filling: Beat together cream cheese, peanut butter, sugar, and egg in a large bowl on high speed until combined well; set aside.

Make brownie batter: Preheat oven to 350 degree F and arrange rack in center. Line 1 (24-cup) mini muffin pan with baking cups. heat together butter and chocolate in a medium saucepan over moderately low heat, stirring until melted. Remove from heat; stir in sugars and extract until combined well. Add eggs, 1 at a time, stirring vigorously after each addition, until combined well. Add flour and salt; stir until just combined.

Divide batter among baking cups. Spoon filling into a small pastry bag fitted with a 1/4-inch plain tip. With tip slightly inserted into center of batter, pipe about 1 teaspoon filling into batter in each cup, allowing a small amount to show on tops. Sprinkle with nuts.

Bake until brownies are puffed and set, 12 to 15 minutes. Transfer pan to rack and let cool 5 minutes. Remove brownies from pan and place on rack to cool completely. (can be made ahead. Store in an airtight container, chilled, up to 2 days. Bring to room temperature before serving.) Makes 24.