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.
To make the pretzel crust: In a medium bowl, mix crushed pretzels and sugar. Add melted butter and mix until evenly moistened. Place in prepared pan and bake for 8-10 minutes until golden. Set aside to cool before adding brownie batter.
To make the brownies: In a large heat-proof bowl, place chocolate and butter. Heat in the microwave in 20- to 30-second intervals, stirring in between each interval, until melted (or set it over a saucepan of simmering water, stirring occasionally, until melted). Add sugar and whisk 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. Fold in pretzels.
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. Top with some more pretzels if you like. Bake for 20 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 moist, not dry. While still in the pan, place brownies on a wire rack to cool completely, then refrigerate until completely 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.