Preheat oven to 350°F (180°C).

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.

In a medium bowl, whisk together flour, baking powder, and salt. With mixer on low speed, add flour mixture to butter mixture in three additions alternately with sour cream, beginning and ending with flour mixture, beating just until combined after each addition. Divide batter in half (about 3 cups or 780 grams each).

In a small bowl, stir together melted chocolate and espresso powder until espresso fully dissolves. Add to 1 portion of batter, and fold until combined. Leave remaining portion of batter as is.

Using 2 (1½-tablespoon) spring-loaded scoops, scoop batters in an alternating pattern into bottom of a 9-inch nonstick tube pan. (See Note.) Firmly tap pan on a kitchen towel-lined counter 2 or 3 times to help level batter and fill pan. Create another layer with alternating cake batters. Tap pan again. Scoop remaining batter randomly on top. Using a butter knife, swirl batters together in a figure eight motion. Tap pan again to help level batter and fill in any gaps. Sprinkle Espresso Streusel on top, making sure to break up any large chunks.

Bake until a wooden pick inserted in center comes out clean, about 1 hour and 5 minutes, loosely covering with foil to prevent excess browning, if necessary. Let cool in pan for 20 minutes. Remove from pan. Best served warm.

In a medium bowl, whisk together flour, sugar, espresso powder, and salt. Using a pastry blender or 2 forks, cut in cold butter until combined and mixture is crumbly. Mix with your hands until completely combined and no more butter lumps remain. (Mixture should resemble wet sand.) Freeze until ready to use.