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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.
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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.
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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.
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Store brownies at room temperature or in the fridge in an airtight container for up to 3 days.