Preheat the oven to 350°F (175°C). Lightly grease a 9-inch heart-shaped cake pan (or a 9-inch round if heart not available) and line the bottom with parchment paper. Lightly flour the pan or dust with cocoa powder.

Dry ingredients

In a medium bowl, sift or whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and fine salt until evenly combined and lump-free.

Wet ingredients

In a large bowl, whisk together the granulated sugar and eggs until combined. Add the whole milk, vegetable oil and vanilla extract and whisk until smooth.

Combine batter

Add the dry ingredients to the wet ingredients in two additions, stirring gently after each addition until mostly combined. Pour in the hot water and whisk just until the batter is smooth and pourable. The hot water thins the batter and blooms the cocoa — it's expected to be loose.

Bake

Pour the batter into the prepared heart-shaped pan and tap gently on the counter to release large air bubbles. Bake in the preheated oven for 28–35 minutes, rotating the pan halfway through for even baking. Cake is done when a toothpick inserted near the center comes out with a few moist crumbs but not wet batter.

Cool

Let the cake cool in the pan for 10–15 minutes. Run a thin knife around the edge, invert onto a wire rack, remove parchment, and allow to cool completely (about 1 hour) before filling or coating. Cooling fully prevents ganache from sliding off.

Filling (optional)

Make filling

In a small bowl, beat the mascarpone (or softened cream cheese) with powdered sugar, cocoa powder and vanilla extract until smooth and spreadable. Chill briefly if too soft (10–15 minutes).

Assemble layer

If you want a layered heart cake, level the cooled cake by slicing the top to make it flat. Slice the cake horizontally into two thin layers using a serrated knife. Spread the cocoa mascarpone filling evenly over the bottom layer, then place the top layer back on gently. Press lightly to adhere.

Chocolate Ganache

Heat cream

Place the chocolate chips in a heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer (bubbles around the edge), then remove from heat.

Make ganache

Pour the hot cream over the chocolate chips. Let sit undisturbed for 1 minute to soften the chocolate. Add the unsalted butter and corn syrup (if using). Stir gently with a spatula from the center outward until the mixture is glossy and smooth. If some chips remain, microwave in 10-second bursts, stirring between, until fully melted.

Cool slightly

Allow the ganache to cool and thicken for 5–10 minutes until it coats the back of a spoon but is still pourable. If it's too thin, cool a bit longer; if too thick, warm gently for a few seconds.

Coat cake

Place the cooled (and filled, if using) cake on a wire rack set over a baking sheet to catch drips. Pour the ganache in the center and use an offset spatula to gently spread it toward the edges, letting it drip down the sides to create a smooth, even coating. Work quickly before ganache sets.

Set

Let the ganache set at room temperature for 30–45 minutes, or chill for 10–15 minutes to speed set. If chilling, bring to room temperature before serving for best texture.

Decoration and Serving

Finish details

Dust the top lightly with unsweetened cocoa powder or sift powdered sugar around the outer edge for contrast. Arrange fresh raspberries or sprinkle edible gold flakes on one side for a romantic touch.

Slice and serve

For clean slices, warm a large chef's knife under hot water, dry it, then make decisive, single strokes through the ganache and cake. Serve each slice at room temperature. Store leftover cake covered in the refrigerator for up to 4 days; bring to room temperature before serving to soften ganache and crumbs.