Preheat the oven to 350°F (175°C). Lightly grease an 8-inch round cake pan and line the bottom with parchment; grease the parchment and lightly flour the sides, tapping out excess.

Combine dry ingredients

In a medium bowl, whisk together the flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until evenly distributed and there are no lumps of cocoa.

Mix wet ingredients

In a separate bowl, whisk the egg with the milk, vegetable oil and vanilla until smooth.

Combine wet and dry

Pour the wet mixture into the dry ingredients and stir with a spatula or whisk until just combined and no dry streaks remain. Batter will be thick.

Add hot water

Stir in the hot water gradually until the batter is smooth and loose—this helps yield a moist, dense, fudgy crumb.

Bake

Pour the batter into the prepared pan and smooth the top. Bake on the center rack for 22–28 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). Rotate the pan halfway through if your oven has hot spots.

Cool

Allow the cake to cool in the pan on a wire rack for 10 minutes. Run a thin knife around the edge, invert onto the rack, remove the parchment, then invert right-side-up onto a serving plate. Cool another 20 minutes so the sauce won’t melt into the crumb excessively.

Dark Chocolate Sauce

Warm cream

Place the chopped chocolate in a heatproof bowl. In a small saucepan, bring the heavy cream just to a simmer over medium heat (small bubbles form around the edge). Do not boil.

Make ganache

Pour the hot cream over the chopped chocolate and let sit for 1 minute to soften. Gently whisk from the center outward until smooth and glossy. Add the butter, corn syrup (if using) and salt, and whisk until fully incorporated.

Adjust consistency

If the sauce is too thick for drizzling, warm gently over a double boiler or microwave in 10-second bursts, stirring between, until pourable. If too thin, let it cool 5–10 minutes to thicken slightly.

Cool slightly before using

Let the sauce rest about 5 minutes off heat so it’s warm but not hot; this keeps the cake from soaking up too much sauce and gives a silky finish.

Finish & Serve

To serve, place the single-layer cake on a dessert plate. Spoon or pour the warm dark chocolate sauce over the center to create a glossy blackout surface that runs toward the edges. Use enough to coat the top and slightly drip down the sides if desired.

Optionally dust the edges with a light sift of powdered sugar and garnish with fresh berries or mint. Serve warm or at room temperature. Leftover sauce can be refrigerated in a sealed container for up to 5 days and gently reheated.