Step 1: Prepare the peanut butter filling. In a small bowl, mix together the peanut butter and confectioners’ sugar until smooth. Roll the mixture into 24 teaspoon-sized balls and place them on a parchment-lined tray. Freeze for at least 1 hour.
Step 2: While the peanut butter filling chills, prepare the brownie cookie dough. Melt the semi-sweet chocolate in a double boiler or microwave in 30-second intervals until smooth. Let it cool slightly.
Step 3: In a separate bowl, whisk together the flour, cocoa powder, baking powder, salt, and espresso powder (if using). Set aside.
Step 4: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.
Step 5: Gradually mix the cooled melted chocolate into the wet ingredients. Then, slowly add the dry ingredients to the chocolate mixture, mixing until just combined. Do not overmix. Cover the dough and refrigerate for at least 1 hour.
Step 6: Preheat your oven to 350°F (177°C). Line two baking sheets with parchment paper. Scoop a tablespoon of the chilled cookie dough, flatten it slightly, and press your thumb into the center.
Step 7: Place one frozen peanut butter ball into the indentation. Gather the brownie dough around the peanut butter ball, ensuring it's completely sealed, and roll into a smooth ball. Place cookies 3 inches apart on the baking sheets.
Step 8: Bake for 12-13 minutes, or until the edges are set but the centers still look slightly soft and fudgy. Let the cookies cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely.