Martha Stewart's Peanut Butter-Filled Chocolate Cupcakes

I made these delicious cupcakes (from "Martha Stewart's Cupcakes") last weekend for a potluck picnic for my sister's 10-year high school reunion. I chose these because they had to travel (and would be outside) and they didn't require any frosting. They turned out pretty good, though I would recommend keeping an eye on them as the recipe calls for a 40-minute bake time and mine were done in about 30.

Peanut Butter-Filled Chocolate Cupcakes (Makes 12)
Ingredients:
2/3 cup all-purpose flour
1/2 teaspoon baking powder
1/4 teaspoon salt
1/2 cup (1 stick) unsalted butter, cut into small pieces
4 ounces semisweet chocolate
2 ounces unsweetened chocolate
3/4 cup granulated sugar
3 large eggs
2 teaspoons pure vanilla extract
Peanut Butter filling

Peanut Butter Filling:
4 tablespoons (1/2 stick) unsalted butter, melted
1/2 cup confectioner's sugar, sifted
3/4 cup smooth peanut butter
1/4 teaspoon salt
1/2 teaspoon pure vanilla extract
Stir together all ingredients until smooth. Use immediately.

1. Preheat oven to 325 degrees. Line a standard muffin tin with paper liners. Whisk together flour, baking powder and salt. Put butter and chocolates in a heatproof bowl set over (not in) a pan of simmering water; stir until melted. Remove from heat, and let cool slightly. (Note - I melted the butter and chocolate in a glass bowl in the microwave at 30-second intervals, stirring in between.)
2. Whisk granulated sugar into cooled chocolate mixture. Add eggs, and whisk until mixture is smooth. Stir in vanilla. Add flour mixture; stir until well incorporated.
3. Spoon 2 tablespoons chocolate batter into each lined cup, followed by 1 tablespoon peanut butter filling. Repeat with another tablespoon of batter, and top with 1 teaspoon filling. Swirl top of cupcake batter and filling with a wooden skewer or toothpick.
4. Bake, rotating tin halfway through, until a cake tester inserted in center comes out with only a few moist crumbs attached, about 40 minutes. Transfer tin to a wire rack to cool completely before removing cupcakes. Cupcakes can be stored up to 3 days at room temperature in airtight containers.