Preheat oven to 350F/175C. Line an 8×8-inch baking pan with parchment paper, leaving enough overhang on the sides for lifting the brownies out after baking.
In a large heat-proof bowl, place chocolate, butter, and instant coffee granules. Heat in the microwave in 20-second intervals, stirring in between each interval, until melted (or set it over a saucepan of simmering water, stirring occasionally). Stir in sugar until combined. Add eggs, one at a time, and whisk just until combined. Add flour and salt and fold, using a rubber spatula, just until combined. Do not overmix.
Pour batter into prepared pan and smooth the top. Spoon dollops of peanut butter on top of brownie batter (small ones, so they won’t sink). Then, using a knife or skewer, gently swirl the two batters together to get a marble effect. Bake for 20-25 minutes, until the edges are set but the center is still very soft. Be careful not to overbake. A toothpick inserted into the center of the brownies should come out with moist crumbs, not dry. Let brownies cool completely on a wire rack, then refrigerate until set for easy cutting. Lift brownies out of the pan using parchment paper and cut into squares.
Store brownies at room temperature or in the fridge in an airtight container for up to 3 days.