Preheat your oven to 350°F (175°C).
Grease a 9-inch round cake pan or springform pan and line the bottom with parchment. Lightly dust the sides with cocoa powder to prevent sticking.
In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Break up any cocoa lumps so the batter mixes smoothly.
In a separate bowl, whisk the eggs, buttermilk, oil, and vanilla until combined.
Stir in the cherry juice or kirsch if using.
Pour the wet ingredients into the dry ingredients. Stir with a spatula until mostly combined. The batter will be thick at this point.
Slowly add the hot coffee or water, whisking gently.
The batter will become glossy and pourable. Do not overmix—a few small lumps are fine.
Pat the cherries dry with a paper towel to reduce excess moisture. Toss them with 1–2 teaspoons of flour to help prevent sinking.
Fold the cherries into the batter with a few gentle strokes. Scrape the batter into the prepared pan and smooth the top.
Bake for 35–42 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.
Avoid overbaking to keep the cake tender.
Cool the cake in the pan for 15 minutes. Run a knife around the edge and invert onto a wire rack. Peel off the parchment and let it cool completely.
For the ganache, heat the cream until steaming (not boiling).
Pour it over the chocolate and let it sit for 2–3 minutes. Add the butter, then stir until smooth and glossy.
When the cake is fully cool, pour the ganache over the top and coax it to the edges so it drips slightly. Let it set for 20–30 minutes before slicing.