In a medium bowl whisk together the dry sponge ingredients: all-purpose flour, cocoa powder, granulated sugar, baking powder, baking soda, and salt until evenly combined.

In a separate bowl or large measuring cup, whisk the wet ingredients: eggs, whole milk, hot brewed coffee, vegetable oil, and vanilla until smooth. The hot coffee helps bloom the cocoa and dissolve sugar — it’s fine if the mixture is warm.

Make a well in the center of the dry ingredients, pour in the wet mixture, and stir gently with a spatula just until combined. Do not overmix — small lumps are okay. Batter will be slightly thin due to oil and coffee.

Spoon the batter into the prepared mini cups, filling each about 2/3 full (should yield 12 cakes). Tap the pan lightly on the counter to release air bubbles.

Bake in the preheated oven 12–15 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter). Rotate the pan halfway through if your oven has hot spots.

Remove from oven and allow the mini cakes to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before frosting (about 30–40 minutes).

Coffee buttercream

Place the softened butter in a mixing bowl. Using a hand mixer or stand mixer fitted with the paddle or whisk attachment, beat the butter on medium speed until smooth and pale, about 1–2 minutes.

Dissolve the instant espresso granules in the heavy cream (or 2 tablespoons hot water if you prefer stronger coffee flavor) and add to the butter.

With the mixer on low, gradually add the confectioners' sugar, about 1/2 cup at a time, scraping down the bowl as needed. Once combined, increase speed to medium-high and beat until light and fluffy, about 2–3 minutes. Add salt and more cream (up to 1–2 tablespoons) as needed to reach a pipeable consistency.

Taste and adjust coffee intensity by adding a pinch more espresso or a small splash of brewed coffee if desired. Transfer the buttercream to a piping bag fitted with a small round or star tip for easier application.

Chocolate ganache

Place chopped chocolate in a small heatproof bowl. Heat the heavy cream in a small saucepan or microwave until just simmering (do not boil). Pour the hot cream over the chocolate and let sit 1–2 minutes to melt.

Stir gently until smooth and glossy. If using, stir in the 1/2 tablespoon unsalted butter for additional shine. Allow the ganache to cool slightly until it thickens to a drizzle-able consistency (about 5–10 minutes).

Assembly and finishing

If the mini cakes have domed tops, level them lightly with a serrated knife so frosting sits evenly.

Pipe about 1 to 1 1/2 tablespoons of coffee buttercream onto each cooled mini cake in a small swirl or dot. Alternatively, spread with a small offset spatula.

Using a spoon or small piping bag, drizzle the slightly cooled ganache over the buttercream on each mini cake. The ganache should form thin ribbons; if too runny, cool a bit longer.

Optional: dust with a pinch of cocoa powder or a little grated chocolate for decoration.

Let assembled mini cakes sit at room temperature for 15–20 minutes to let the ganache set slightly before serving. Store in an airtight container at cool room temperature for 1 day or refrigerate up to 3 days (bring to room temperature before serving).