Preheat your oven to 180°C (356°F) with both top and bottom heat for about 10 minutes. This ensures the crust crisps up evenly and quickly.

In a small bowl, combine the 1 packet of instant coffee powder with 50-60ml of hot water. Stir vigorously until the coffee is completely dissolved. Set this aside and allow it to cool to room temperature. This is a crucial step to prevent the hot coffee from cooking the egg in the next steps.

In a medium-sized mixing bowl, combine the 60g of softened butter and 50g of powdered sugar. Use a silicone spatula to mix and press them together until the mixture is smooth, uniform, and slightly paler in color.

Add the 1 room-temperature egg to the butter-sugar mixture. Continue to mix until the egg is fully incorporated and the mixture looks creamy, with no signs of separation.

Sift the 70g of cake flour directly into the bowl. Use the spatula to fold the flour into the wet ingredients. Mix until just combined and no dry streaks of flour remain; be careful not to overmix.

Once the coffee liquid has cooled, pour it into the batter. Stir until everything is fully combined, resulting in a thick, spreadable, and smooth coffee paste. Pour the coffee paste into a piping bag.

Lightly brush one side of each white bread slice with a thin layer of softened butter — this adds extra flavor and prevents sogginess.

On the opposite side, pipe the coffee paste evenly and spread it gently with the back of a spoon.

Arrange the bread on a baking tray lined with parchment paper.

Bake in the preheated oven at 180°C (356°F) for 18 minutes, or until the tops are golden brown and crisp with slightly caramelized edges.

Remove from the oven and cool on a wire rack for 2 minutes. Enjoy warm while the crust is still crispy!