Make the brownie cookie dough: Melt the chocolate in a double boiler or use the microwave. Microwave in 20-second increments, stirring after each until completely melted. Set aside to slightly cool.
Meanwhile, whisk the flour, cocoa powder, espresso powder, baking powder, and salt together. Set aside. In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar together on medium-high speed until smooth and creamy, about 2 minutes. Add the eggs and vanilla extract, and beat on high speed for 2 full minutes. Scrape down the sides and up the bottom of the bowl, then beat on high for 1 more minute. Pour in the slightly cooled melted chocolate and mix on medium-high speed for 2 full minutes. Pour the dry ingredients into the wet ingredients and beat on low speed until combined.
Cover and chill the dough in the refrigerator for 1 hour. If chilling for longer, allow the cookie dough to sit at room temperature for 10-15 minutes before rolling and shaping in step 6.
After chilling, preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats.
Shape the cookies: The peanut butter filling softens up extremely quickly after you take it out of the freezer. I recommend taking about 6 peanut butter balls out of the freezer at a time, leaving the rest in the freezer while you’re shaping the first bunch. Using a sturdy tablespoon, scoop out a heaping spoonful of the cookie dough and roll into a ball, about 1.5 Tablespoons/30g/1 ounce of dough each. The dough will be quite stiff. Make an indentation with your thumb, and place a frozen peanut butter ball in the indentation. Pinch off a piece of the brownie cookie dough and press it on top of the peanut butter filling, and then roll the whole thing together into a smooth ball. Feel free to pull off pieces of brownie dough to expose more peanut butter and then re-roll together; peanut butter can also be hidden inside the cookie dough if you don’t want a marbled look. Repeat with remaining cookie dough and peanut butter balls. (Warning: your hands get messy in this step!)
Place dough balls 3 inches apart on the baking sheets and bake for 12-13 minutes or until the edges appear set. The centers will be quite soft, but will set up as the cookies cool.
Remove from the oven and allow cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.
Cookies stay fresh covered at room temperature for up to 1 week.
