Preheat the oven to 350°F (177°C). Grease a 17 × 12 inch (43 × 30 cm) rimmed baking sheet and line it with parchment paper, letting a bit hang over the edges for easy lifting.

In a large bowl, whisk together the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder until evenly combined. In a separate bowl, whisk together the oil, eggs, buttermilk, and vanilla until smooth. Pour the wet ingredients into the dry ingredients, add the hot coffee, and whisk until the batter is smooth and glossy (it will be thin).

Pour the batter into the prepared pan and spread evenly to the edges. Bake for 18 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan on a wire rack.

Meanwhile, stir the sugar into the hot coffee until dissolved and set aside.

In a stand mixer fitted with the paddle attachment, beat the butter on medium speed until pale and fluffy, about 2 minutes. Reduce speed to low, gradually add the icing sugar, and mix until mostly combined. Add the vanilla, cocoa powder, and salt, then increase speed to medium-high and beat until light and creamy, about 2 minutes more.

Lift the cooled cake from the pan using the parchment overhang. Using your coupe glass as a cutter, stamp out circles of cake and press one into the bottom of each glass. Use a fork to poke a few holes in the surface, then spoon 1–2 teaspoons of the coffee soak over each to moisten.

Spread or pipe frosting over each cake layer, smoothing it level with the rim of the glass. Dust lightly with cocoa powder and top with three coffee beans.