Preheat the oven to 350° F (180° C.) Prepare an 8x8 inch (20 cm) baking pan with parchment paper or cooking spray.
In a large mixing bowl, stir together the salt and sugar. Add the melted butter and liqueur or extract and stir until well combined. Next, add the eggs one at a time, stirring until each one is fully incorporated.
Next, add the cocoa powder and stir until it is well combined. Add the flour last, stirring until it is just combined, being careful not to overmix.
Pour the batter into the prepared pan. Place spoonfuls of the peanut butter evenly over the surface of the batter, then use a knife or toothpick to create a swirl pattern.
Bake at 350° F (180° C) for 45-50 minutes, or until the center of the brownies no longer wobbles. Store at room temperature for 3-4 days, or a little longer in the refrigerator.
