Prep the pan. Heat the oven to 350 degrees F, grease a 9x9 inch pan, and line it with a parchment sling.
Melt the chocolate. Microwave the chocolate and butter at half power in 30 second pops, stirring until smooth.
Make the batter. Whisk in the cocoa, both sugars, eggs, vanilla, espresso powder, flour, and salt, then fold in the peanut butter cups and chips.
Make the swirl. Stir the melted butter, peanut butter, powdered sugar, and vanilla in a separate bowl until just combined.
Layer and swirl. Spread two thirds of the batter in the pan, dollop on the filling, cover with the rest of the batter, and swirl once or twice with a butter knife. Top with more cups and chips.
Bake. Bake for 30 to 35 minutes, until a toothpick shows a few moist crumbs, then cool for 1 hour before cutting.