Preheat the oven to 350°F (175°C) and grease an 8-inch springform pan or line it with parchment paper.

In a bowl, whisk together the flour, cocoa, sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk eggs, milk, oil, and vanilla until smooth. Gradually add the dry ingredients, then stir in the hot water until the batter is thin and uniform.

Pour the batter into the prepared pan and bake for 20–25 minutes, or until a toothpick comes out clean. Allow the cake to cool completely on a wire rack.

Chop the dark chocolate and place it in a heatproof bowl.

In a small saucepan or microwave, warm a small amount of heavy cream until hot but not boiling. Pour it over the chocolate and let sit 1–2 minutes, then stir until fully melted and glossy.

Stir in orange juice, orange zest, and espresso or coffee (if using).

In a separate chilled bowl, whip the remaining heavy cream with powdered sugar and vanilla until soft peaks form.

Fold about one-third of the whipped cream into the chocolate mixture to lighten it, then gently fold in the remaining whipped cream until smooth and airy.

Once the cake base is completely cool, place it on a serving plate or cake board.

Pour the prepared mousse over the cake base in an even layer. Smooth the top with a spatula.

Refrigerate for at least 4 hours, preferably overnight, so the mousse fully sets and the flavors meld together.

For a polished finish, drizzle a dark chocolate ganache over the set mousse layer.

Decorate with candied orange peel.

Dust with cocoa powder.

Arrange fresh orange segments around the edge.