Preheat oven to 350°F/175°C. Line a 9x13 inch baking pan with foil and grease lightly. Sift together flour and cocoa powder in a bowl and set aside.
Melt butter over low heat, then increase to medium and simmer. Continue cooking until it has browned, stirring constantly. Remove from heat and stir in chocolate, until chocolate has melted.
Combine sugars, salt, eggs, vanilla and espresso powder in a stand mixer with a whisk. Whip on medium/high until thick, about 8 minutes.
Reduce to low and pour in chocolate butter mix. Make sure it is completely incorporated.
Add flour/cocoa powder mixture and mix until well combined. Stop mixer, and using a flexible spatula fold in pretzels.
Pour into prepared pan and bake for 25 minutes. Brownies should be glossy and firm. Cool to room temp.