10 ounce bittersweet chocolate, chopped
2 ounces unsweetened chocolate, chopped
1/4 cup unsalted butter, cut into chunks
3 large eggs, at room temperature
1 cup sugar
3/4 cup flour
3/4 teaspoon baking powder
1/4 teaspoon kosher salt
1/2 cup peanut butter
1. Put chocolates and butter in a medium metal bowl and set bowl over a pan filled with 1 in. of simmering water. Cook, stirring occasionally, until melted, then remove from heat and let cool slightly. Whisk in eggs and sugar, mixing until combined. Then whisk in flour, baking powder, and salt. Chill dough, covered, until firm, about 2 hours.
2. Let dough sit at room temperature 15 minutes. Meanwhile, preheat oven to 350° and line 2 baking sheets with parchment paper. Scoop 1 tbsp. portions of dough, rolling each into a ball, and put onto sheets 1 in. apart. Press your thumb into the center of each cookie ball, making a small well. Fill a resealable plastic bag with peanut butter. With scissors, snip off 1 corner of bag and squeeze about 1/2 tsp. peanut butter from bag into each well.
3. Bake cookies until they no longer look wet on top, about 8 minutes. Let cool on baking sheets.
Make ahead: Batter up to 1 day; baked cookies up to 2 days, airtight.