In a bowl, whisk together flour, baking powder, and salt; set aside.

In a separate bowl, cream butter and sugar until light and fluffy (about 2–3 minutes).

Mix in egg yolk, vanilla extract, and lemon zest (if using) until incorporated.

Gradually add the flour mixture, mixing until a soft dough forms.

Form dough into a disk, wrap in plastic, and chill for 30 minutes.

Preheat oven to 350 °F (175 °C). Line a baking sheet with parchment paper.

Roll dough into 1‑in balls, place 1 in apart on sheet. Gently flatten each ball to about ½‑in thick.

Bake for 10–12 minutes until edges are lightly golden. Let cool on the sheet for 5 minutes, then transfer to a wire rack.

Once completely cool, brush the top of each cookie with a thin layer of reserved egg white.

Sprinkle about 1 tsp of granulated sugar on top of each cookie, covering evenly.

Using a kitchen torch (or broiler on low), carefully melt and caramelize the sugar until golden and crisp—watch closely to avoid burning.

Let cookies sit for a few minutes until the sugar shell sets, then serve.