Preheat the oven to 350°F (175°C) and prepare a 9-inch round cake pan by lining it with parchment paper and non-stick spray.

In a mixing bowl, whip the cold egg whites until foamy, then gradually add half of the granulated sugar until stiff peaks form.

In another bowl, whisk egg yolks and remaining sugar until pale; stir in cooled melted butter, lemon zest, and lemon extract.

Sift together the almond flour, cornstarch, baking powder, and salt; fold into the yolk mixture until a thick batter forms.

Gently fold in the whipped egg whites in three additions until no white streaks remain.

Pour batter into the prepared pan, optionally topping with sliced almonds and turbinado sugar.

Bake for 30-34 minutes; it's done when a toothpick inserted comes out clean.

Cool in the pan for 10 minutes, transfer to a wire rack, then drizzle with glaze made of powdered sugar and lemon juice.