For the batter:
½ c. unsalted butter, cut into small pieces, plus more for the pan
2 oz. unsweetened chocolate, coarsely chopped
4 oz. semisweet chocolate, coarsely chopped
2/3 c. all-purpose flour
½ tsp. baking powder
¼ tsp. coarse salt
¾ c. granulated sugar
3 large eggs
2 tsp. pure vanilla extract
for the filling:
4 TB unsalted butter, melted
½ c. confectioners’ sugar
¾ c. smooth peanut butter
¼ tsp. coarse salt
½ tsp. pure vanilla extract
1. Preheat oven 325. Butter an 8 inch square pan and line with parchment, allowing 2 inch overlap. Butter linging.
2. Make batter: put butter and chocolates in a heatproof medium bowl set over a pan of simmering water, stir until melted. Let cool slightly. Whisk together flour, baking powder and salt in bowl.
3. Whisk granulated sugar into chocolate mixture. Add eggs and whisk until mixture is smooth. Stir in vanilla. Add flour mixture; stir until well combined.
4. Make filling: stir together butter, confectioners’ sugar, peanut butter, salt and vanilla until smooth.
5. Pour one-third of batter into prepared pan; spread evenly with a rubber spatula. Drop dollops of peanut butter filling (about 1 TB each) on top of batter, spacing about 1 inch apart. Drizzle remaining batter on top, and gently spread to fill pan. Drop dollops of peanut butter on top. Gently swirl filling into batter with a knife.