This rich and delicious cake not only holds up for days--it gets even better. Baked with both instant powdered and brewed espresso, it boasts subtle coffee flavor. Sour cream in the batter ensures a moist texture and enhances taste.
Filling: Preheat oven to 325 degrees. Stir together cocoa, espresso powder, and granulated sugar.
Cake: In a small bowl, mash together 2 tablespoons butter and 1 tablespoon cocoa; generously brush a 12-cup nonstick Bundt pan with mixture. In another bowl, whisk together flour, salt, baking soda, and baking powder. In another bowl, whisk brewed espresso into remaining 1/2 cup cocoa and espresso powder until smooth; whisk in sour cream.
With a mixer on medium-high speed, beat remaining 3 sticks butter with granulated sugar until light and fluffy, about 2 minutes. Add eggs, one at a time, beating well after each addition. Reduce speed to low and add flour mixture in three batches, alternating with cocoa mixture in two batches. Beat to combine, scraping down bowl as needed. Beat in chopped chocolate.
Transfer one-third of batter to prepared pan. Sprinkle with half of filling, avoiding edges. Repeat, then finish with remaining one-third of batter, smoothing top with an offset spatula. Bake until a tester inserted in center comes out clean, about 1 hour. Let cool in pan on a wire rack 15 minutes, then turn cake out onto rack; let cool completely, at least 2 hours.
Dust with confectioners' sugar. Serve, with a dollop of whipped cream and some candied lemon zest.