Cream together the French butter and granulated sugar until pale and fluffy, about 3-5 minutes.

Add the egg and vanilla extract; mix on low speed until smooth, about 1-2 minutes.

In a separate bowl, whisk together the all-purpose flour, baking powder, and sea salt.

Gradually add the dry mixture to the wet ingredients, stirring gently until just combined.

Wrap the cookie dough in plastic wrap and refrigerate for at least 1 hour.

Roll out the chilled dough on a floured surface to about 1/2-inch thickness and cut into cookies.

Brush the tops of the cut cookies with egg yolk for a glossy finish.

Bake in a preheated oven at 350°F (175°C) for 15-18 minutes until golden.

Cool on the tray for a few minutes, then transfer to a wire rack. Sprinkle with flaky sea salt while warm.