Prep the pan and oven. Heat the oven to 350°F (175°C). Brush the bundt pan with softened butter, getting into every nook. Dust lightly with cocoa powder, tapping out the excess. This prevents white flour streaks on a dark cake.

Whisk the dry ingredients. In a large bowl, whisk flour, cocoa powder, baking powder, baking soda, and salt until evenly combined. Break up any cocoa lumps with the whisk.

Mix the wet ingredients. In another bowl, whisk sugars, eggs, oil, sour cream, and vanilla until smooth. Stir in the melted chocolate. The mixture should look glossy and unified.

Alternate buttermilk and dry mix. Add one-third of the dry ingredients to the wet mixture, then half the buttermilk. Repeat, ending with the dry ingredients. Mix gently with a spatula until just combined. Do not overmix.

Add hot coffee. Pour in the hot coffee and fold until the batter is smooth. It will be thinner now; that’s good. Hot liquid helps bloom the cocoa for fuller flavor.

Fill the pan. Scrape the batter into the prepared bundt pan. Tap the pan firmly on the counter 2–3 times to remove air bubbles. Smooth the top with a spatula.

Bake. Bake for 45–55 minutes, or until a skewer comes out with a few moist crumbs. The cake should spring back when lightly pressed.

Cool carefully. Let the cake cool in the pan for 15–20 minutes. Run a thin offset spatula around the center tube and outer edge if needed. Invert onto a wire rack and lift off the pan. Cool completely before glazing.

Make the glaze. Heat cream until steaming. Pour over chopped chocolate with the pinch of salt and optional corn syrup. Let sit 2 minutes, then stir until smooth and shiny.

Glaze the cake. Set the cooled cake on a rack over a sheet of parchment. Pour glaze slowly over the top, letting it cascade down the sides. Let set 20–30 minutes before slicing.