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1
FOR THE PAN: Stir together butter and cocoa in small bowl until paste forms; using a pastry brush, coat all interior surfaces of standard 12-cup Bundt pan, see illustration below. (If mixture becomes too thick to brush on, microwave it for 10 to 20 seconds, or until warm and softened.) Adjust oven rack to lower-middle position; heat oven to 350u00b0F.
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FOR THE CAKE: Combine cocoa, chocolate, and espresso powder (if using) in medium heatproof bowl; pour boiling water over and whisk until smooth. Cool to room temperature; then whisk in sour cream. Whisk flour, salt, and baking soda in second bowl to combine.
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3
In standing mixer fitted with flat beater, beat butter, sugar, and vanilla on medium-high speed until pale and fluffy, about 3 minutes. Reduce speed to medium and add eggs one at a time, mixing about 30 seconds after each addition and scraping down bowl with rubber spatula after first 2 additions. Reduce to medium-low speed (batter may appear separated); add about one third of flour mixture and half of chocolate/sour cream mixture and mix until just incorporated, about 20 seconds. Scrape bowl and repeat using half of remaining flour mixture and all of remaining chocolate mixture; add remaining flour mixture and beat until just incorporated, about 10 seconds. Scrape bowl and mix on medium-low until batter is thoroughly combined, about 30 seconds.
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4
Pour batter into prepared Bundt pan, being careful not to pour batter on sides of pan. Bake until wooden skewer inserted into center comes out with few crumbs attached, 45 to 50 minutes. Cool in pan 10 minutes, then invert cake onto parchment-lined wire rack; cool to room temperature, about 3 hours. Dust with confectioners' sugar, transfer to serving platter, and cut into wedges.
