Preheat the oven to 350°F (175°C). Line a mini muffin tin (or an 8x8-inch square pan for slightly larger snack-size squares) with paper liners or lightly grease the cavities so the mini mocha cakes release easily.

Combine dry ingredients

In a medium bowl, whisk together 1 cup all-purpose flour, 3 tablespoons granulated sugar, 2 tablespoons unsweetened cocoa powder, 1 teaspoon instant espresso powder, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon fine salt until evenly distributed.

Mix wet ingredients

In a separate small bowl or measuring cup, beat 1 large egg lightly, then whisk in 1/2 cup whole milk, 2 tablespoons neutral oil, and 1/2 teaspoon vanilla extract until combined.

Make batter

Pour the wet ingredients into the dry ingredients and stir with a spatula or wooden spoon just until combined and no large streaks of flour remain. Do not overmix; a few small lumps are fine. Batter will be slightly thick but pourable. If it seems too thick, add 1 tablespoon milk.

Portion and bake

Spoon the batter into the prepared mini muffin tin, filling each cavity about 3/4 full (this recipe makes approximately 12 mini cakes depending on size). For an 8x8 pan, spread evenly to about 1/2-inch thickness. Bake in the preheated oven for 10–12 minutes for mini muffins, or 16–20 minutes for an 8x8 pan, until a toothpick inserted in the center comes out with a few moist crumbs. Rotate the pan halfway through baking for even color.

Cool

Remove the mini cakes from the oven and allow them to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before glazing (warm cakes will melt the glaze).

Mocha glaze / topping

Heat coffee and melt chocolate

Place 60 g semisweet chocolate chips and 1 tablespoon unsalted butter in a heatproof bowl. Pour 2 tablespoons hot strong brewed coffee or freshly pulled espresso over the chocolate. Let sit 1 minute to soften the chocolate, then stir until smooth. If chocolate appears too thin and you want a thicker glaze, whisk in up to 1 tablespoon powdered sugar until desired consistency is reached.

Glaze the cakes

Once the mini mocha cakes are completely cool, spoon or dip the top of each cake into the warm mocha glaze so that the top is evenly coated. Let excess drip back into the bowl and place cakes on a wire rack set over parchment to catch drips.

Finish and garnish

While glaze is still tacky, sprinkle a tiny pinch (about 1/16–1/8 tsp) of coarse sea salt on each cake and, if using, dust lightly with 1/2 teaspoon cocoa powder or sprinkle chopped toasted nuts. Allow glaze to set at room temperature for about 20–30 minutes, or refrigerate 10 minutes to speed setting.

Serving and storage

Serve the Mocha Cake Minis at room temperature as a snack or with coffee. Store leftovers in an airtight container at room temperature for up to 2 days, or refrigerate for up to 5 days. If refrigerated, bring to room temperature before serving for best texture.