In a medium bowl, whisk together the egg white, granulated sugar, brown sugar, melted butter, and vanilla extract until smooth and slightly foamy.

Add the flour, cocoa powder, salt, and baking powder. Stir gently with a spatula until just combined — the batter will be thick and glossy.

Spread the batter very thinly (about 1/8 inch / 2–3 mm thick) onto a parchment-lined baking sheet. Try to keep it even for a uniform bake.

Place the tray in the fridge for 15–20 minutes to help the batter firm up slightly before baking.

Meanwhile, preheat the oven to 340°F (170°C).

Bake for 15–18 minutes, until the edges are crisp and the center is set.

Remove from the oven and immediately decorate with your chosen toppings — chopped nuts, Christmas sprinkles, or flaky sea salt — while the surface is still warm.

Let it cool completely, then break into shards or pieces.