Prep the pan: Preheat oven to 325°F (165°C). Generously grease and flour a 10-cup Bundt pan, or grease and dust with cocoa powder for a neat chocolate finish.
Bloom cocoa: In a small bowl, whisk the hot coffee (or water) with the cocoa powder and espresso powder (if using) until smooth. Let cool slightly.
Cream butter & sugar: In a large mixing bowl, beat the softened butter and granulated sugar together until light and fluffy, 2–3 minutes.
Add eggs & vanilla: Beat in the eggs one at a time (or 2 whole + 1 yolk for extra richness), then mix in the vanilla.
Mix dry ingredients: In a separate bowl, whisk together flour, baking soda, and salt.
Combine wet & dry: Add ⅓ of the dry mixture to the butter mixture, then half the sour cream. Repeat with another ⅓ of dry, the rest of the sour cream, and the final ⅓ of dry ingredients. Mix gently, just until combined.
Add cocoa mixture: Pour in the bloomed cocoa mixture and stir until the batter is smooth and glossy.
Bake: Spread the batter evenly into the prepared Bundt pan. Bake for 45–55 minutes, or until a toothpick inserted in the deepest part comes out clean or with a few moist crumbs.
Cool & glaze (optional): Let the cake cool in the pan for 10–15 minutes before turning out onto a wire rack. For the glaze, heat cream and butter until steaming, pour over chopped chocolate, let sit 1–2 minutes, then stir until smooth. Drizzle over cooled cake. Once set, top with powdered sugar if desired.