In medium bowl, whisk together the flour, cocoa powder and salt. Whisk to break up any lumps in the cocoa powder.
¾ cup all purpose flour, 2 tablespoons cocoa powder, ½ teaspoon fine sea salt
In a medium saucepan over low heat, melt the butter and espresso powder. Remove from the heat.
½ cup unsalted butter, ¾ teaspoon espresso powder
Stir in the chocolate chips until fully melted and smooth. If there is chocolate that hasn’t melted, put back on low heat and stir until melted.
⅔ cup chocolate chips, bittersweet or semi-sweet
Whisk in the granulated sugar and brown sugar.
¾ cup granulated sugar, ¼ cup light brown sugar, packed
Whisk in the eggs, one at a time, stirring after each addition. Whisk in the vanilla.
3 large eggs, room temperature, 1 teaspoon pure vanilla extract
Fold in the flour mixture, stirring until just combined. Scrape down the sides and bottom of the bowl with a spatula to ensure everything is fully mixed.
Fold in half of the chopped peanut butter cups.
⅔ cup peanut butter cups, chopped
Spread the brownie batter into the prepared pan in an even layer.
Warm the peanut butter in the microwave for 20-30seconds until it has softened and thinned. It should be slightly liquidy and have the consistency of caramel sauce.
Dollop spoonfuls of the peanut butter over the brownie batter. Create swirls in the batter with a toothpick.
¼ cup peanut butter
Sprinkle the remaining half of the chopped peanut butter cups on top of the brownies.
Bake for 25-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let the brownies cool to room temperature, or just warm, then cut into squares and serve.