Fudgy Peanut Butter Cup Brownies

cup unsalted butter (1 stick)

 ounces dark or bittersweet chocolate, chopped (I used Trader Joe’s 72% Pound Plus Bar)

 large eggs

granulated sugar

 tablespoon vanilla extract

brewed coffee (leftover or cold coffee is okay), optional but recommended

instant espresso granules, optional but recommended

 cup all-purpose flour

teaspoon salt, or to taste

full-size peanut butter cups, unwrapped