Preheat the oven to 350℉ and line a 9x13-inch baking pan with parchment paper.
Add the butter and chopped dark chocolate to a medium saucepan. Melt over low heat, stirring gently until smooth.
Stir in the cocoa powder and whisk until fully dissolved into the warm chocolate mixture. Remove from heat, then stir in the sugar and vanilla extract until combined.
Add the eggs one at a time, whisking well after each addition until the batter looks glossy and smooth.
Fold in the flour, instant espresso powder, and dark chocolate chips until just combined. Lightly crush half of the pretzels and fold them into the batter.
Pour the batter into the prepared baking pan and spread it evenly. Arrange the remaining pretzels on top, pressing them lightly into the surface.
Bake for 40-45 minutes, until the brownies are set around the edges but still slightly soft in the center. Start checking at 35 minutes. A toothpick inserted in the center should come out with moist crumbs, not wet batter.
Let the brownies cool completely in the pan before slicing into squares and serving.