For the Ganache Filling:

1. Chop the chocolate finely. Scald the heavy cream and pour it over the chocolate. Stir until smooth and add the liqueur if using. Set aside or refrigerate to solidify. Make this 12–24 hours ahead so it has time to firm up.

For the Cupcakes:

1. In a large bowl, melt together the chocolate, butter, and water (microwave or double boiler). When the water is steamy, remove from heat and whisk to combine. Whisk in the espresso powder and cocoa powder until smooth. Set aside to cool slightly.
2. Preheat the oven to 350°F and line two muffin pans (24 cups total) with cupcake liners.
3. In a medium bowl, mix together the flour, granulated sugar, brown sugar, salt, and baking soda.
4. Into the cooled chocolate mixture, whisk in the yogurt, then the eggs and yolks one at a time, and the vanilla extract. When incorporated, slowly fold in the flour mixture until completely combined — do not overmix.
5. Scoop the batter into the cupcake pans. Scoop out about a tablespoon of hardened ganache and place it in the centre of each cupcake. Bake for 18–20 minutes, or until a toothpick inserted into the side (not the ganache centre) comes out clean. Meanwhile, prepare the buttercream.

For the Coffee Buttercream:

1. Mix the espresso powder into the brewed espresso and set aside to cool completely.
2. In the bowl of a stand mixer, whisk together the egg whites, sugar, and salt. Set the bowl over a saucepan of simmering water and stir occasionally until the mixture is hot and the sugar has dissolved, about 7–9 minutes.
3. Remove from the heat and fit onto the stand mixer with the whisk attachment. Beat on medium, then high speed until stiff, glossy peaks form and the bowl is cool to the touch, about 10 minutes.
4. With the mixer running, add the softened butter a tablespoon at a time over at least 15 minutes — this is how the meringue becomes buttercream and the temperature slowly drops.
5. Add the cooled espresso on low speed, then beat on medium-high until smooth and creamy. (If it looks curdled, keep beating — it will come back together.)
6. Once the cupcakes are completely cool, pipe the buttercream with a large 2D tip, starting from the centre and spiralling outward for a rose effect. Leftover buttercream keeps refrigerated or frozen; bring back to room temperature and re-beat before using.