Preheat the oven to 350°F (175°C). Grease and line a 9-inch round cake pan that is at least 2 inches deep with parchment paper.

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

Add the buttermilk, vegetable oil, eggs, and vanilla extract, then mix until well combined.

Slowly stir in the hot water or coffee until fully incorporated; the batter will be thin.

Pour the batter into the prepared pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

In a saucepan, heat the heavy cream over medium heat until it starts to simmer, but do not let it boil.

Remove from heat and pour over the chopped dark chocolate. Let it sit for 2-3 minutes, then stir until smooth. Add the butter and stir until fully incorporated. Let the ganache cool slightly until it thickens but remains pourable.

Melt the dark chocolate in a microwave-safe bowl using 30-second intervals, stirring between each, until smooth.

Transfer the melted chocolate to a piping bag or a small ziplock bag with the corner snipped off. Lay a sheet of parchment paper on a baking tray, then pipe the chocolate in crisscrossing strands to form a circular nest shape. Place the tray in the refrigerator for 15-20 minutes, or until the chocolate nest is firm.

Place the cooled cake on a serving platter. Pour the chocolate ganache over the cake, letting it drip down the sides.

Carefully peel the chocolate nest from the parchment paper and place it on top of the cake. Fill the nest with mini chocolate eggs before serving.