Preheat your oven to 120°C (250°F). Take your store-bought sponge cake and slice it into ½-inch thick pieces using a serrated knife. Line a baking sheet with parchment paper and arrange the cake slices on it, leaving small gaps between each piece.

Melt your butter in a small bowl. Using a pastry brush, generously coat each slice of cake with the melted butter. Sprinkle white sugar evenly over the buttered surfaces, making sure each piece is well covered.

Place the baking sheet in the preheated oven and bake for 1 hour. Halfway through baking, rotate the pan to ensure even browning. The biscocho is done when the pieces are golden brown and crispy.

Remove from the oven and let the pieces cool on the baking sheet for 5 minutes. Then transfer them to a wire rack to cool completely - they will continue to crisp up as they cool. Once fully cooled, serve with hot coffee or store in an airtight container for up to 2 weeks.

For best results, use day-old cake as it slices more cleanly and absorbs the butter better. If your biscocho isn't crispy enough after cooling, you can return it to the oven for an additional 10-15 minutes at 110°C. And remember, the butter should be warm but not hot when brushing onto the cake slices.