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1
Adjust oven rack to middle position and preheat oven to 350u00b0F. Grease two 9-inch cake pans and line with parchment paper.
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2
Place chocolate in a large bowl and pour coffee over. Whisk until melted and then cool briefly. Whisk in brown sugar, granulated sugar, salt, eggs, oil, sour cream, and vanilla until combined.
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3
Whisk flour, cocoa, baking soda, and baking power in a medium bowl. Whisk into wet mixture until smooth. Evenly divide batter between pans and bake until just set and a toothpick inserted into the center comes out with moist crumbs, 35 to 40 minutes. Let cakes cool in pans for 30 minutes, then invert onto wire racks to cool completely.
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4
Mix butter, chocolate, sour cream, cocoa, and vanilla in mixer with paddle on low speed until combined. Add sugar and mix until just combined. Increase speed to medium and beat until creamy, about 1 minute. If frosting is too soft to spread onto cake, chill briefly until thickened.
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5
Place one cake on serving plate and spread with about 2/3 cup frosting. Sprinkle almost half of the crushed pretzels on the frosting and then place the second cake on top (rounded side up). Use remaining frosting to ice top and sides of cake. Sprinkle remaining pretzels on top.
