Prep your pan and oven. Heat oven to 350°F. Use a 10 to 12-cup Bundt pan (often a 10-inch Bundt). Grease and flour it really well, getting into every groove. If using rounds, grease, line bottoms with parchment, then grease again. Use two 9-inch rounds or three 8-inch rounds for a comfortable fill.
Mix the dry ingredients. In a bowl, whisk flour, baking powder, baking soda, and salt. Set aside.
Bloom the cocoa. In a small bowl, whisk the cocoa powder with the hot coffee (or hot water) until smooth and glossy, like a loose pudding. Let it sit for 2 minutes while you start the batter. This is where the chocolate flavor wakes up.
Cream butter and sugars. In a stand mixer (or with a hand mixer), beat butter, granulated sugar, and brown sugar on medium-high for 3 to 4 minutes, until lighter and fluffy. This step is not just busywork. It sets up the texture.
Add eggs and vanilla. Beat in eggs one at a time, scraping the bowl as needed. Mix in vanilla.
Add sour cream. Beat in sour cream until smooth. The batter may look extra luscious here. That is correct.
Add the bloomed cocoa. Pour in the cocoa-coffee mixture and mix on low just until the batter turns evenly chocolatey.
Alternate dry ingredients and milk. With the mixer on low, add the dry ingredients in three additions, alternating with the milk in two additions (dry, milk, dry, milk, dry). Mix just until you do not see streaks of flour. The batter will be smooth and fairly fluid.
Bake. Pour batter into the prepared pan and smooth the top. For a Bundt, bake 45 to 55 minutes. For two 9-inch rounds, bake 28 to 35 minutes. For three 8-inch rounds, bake 22 to 30 minutes.
Doneness cues: A toothpick should come out with a few moist crumbs, the top should spring back lightly, and the cake should start to pull from the edges.
Cool. Let the cake cool in the pan for 15 minutes, then invert onto a wire rack to cool completely before glazing. If you glaze warm cake, the glaze will slide right off. Delicious, but chaotic.
Make the glaze. Put chopped chocolate in a heat-safe bowl. Heat cream and butter until steaming and just barely simmering, then pour over chocolate. Let sit 2 minutes, then whisk until smooth. Whisk in corn syrup or honey (if using) and a pinch of salt.
Glaze and set. Pour glaze over the cooled cake, letting it drip naturally. Let set for 20 to 30 minutes. Finish with flaky salt or shavings if you want that elegant restaurant vibe.
Slice and serve. Use a sharp knife, wipe between cuts, and serve with berries, whipped cream, or ice cream. Or nothing. This cake can handle standing on its own.
