Heat oven to 350 degrees F. Coat a 15-indent mini cake pan or 15 indents in two mini muffin pans with nonstick cooking spray.
Microwave chocolate in glass bowl for 30 seconds. Stir. Microwave another 30 seconds. Stir until smooth. Set aside.
In small bowl, whisk flour, baking powder and salt. In medium bowl, beat butter until smooth. Beat in sugar until blended, then beat in egg. Beat in slightly cooled melted chocolate. On low speed, beat in flour mixture and then milk.
Transfer batter to a resealable plastic bag. Snip off corner and pipe batter into prepared pan(s). Bake at 350 degrees F for 14 to 15 minutes. Remove cakes from pan(s) directly to rack to cool. Trim rounded tops level; return to pan.
Beat heavy cream, peanut butter and sugar in a small bowl until smooth. Transfer to a disposable plastic pastry bag. Snip off point about 1/2 inch from tip. Poke a hole in cakes with a chopstick. Insert pastry bag into holes and gently squeeze to fill.
Invert cakes onto a serving platter.
Heat cream in microwave for 30 seconds or until just bubbly. Pour over chocolate in small bowl and let stand for 5 minutes. Stir until smooth. Spoon about 2 teaspoons chocolate mixture over each cake, allowing topping to drip over sides. Sprinkle with chopped nuts. Refrigerate until serving.