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.
Marshmallow Meringue
Fill a medium saucepan with an inch of water, and place over medium/low heat.
In the bowl of a stand mixer, combine egg whites, sugar, salt, cream of tartar and vanilla. Place over water, whisking constantly until it goes from a yellowish mixture to white and opaque. This takes about 10 minutes
You'll know it is ready by using a candy thermometer, you should have a reading of 175°. If you don't have a candy thermometer, rub a bit of the mixture with your fingers. You shouldn't be able to feel any sugar granules.
Place the bowl in a stand mixer and whip on high until it is glossy and thick, about 5 minutes. You should have firm peaks.
Spread on cooled brownies. Use an offset spatula to create swoops and swirls. Use a blowtorch to toast the marshmallow.