In a large mixing bowl, use an electric mixer to beat the French butter with granulated sugar until pale and fluffy, about 4-5 minutes.

Add the egg and vanilla extract to the creamed mixture, mixing on low speed until smooth, about 1-2 minutes.

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

Gradually add the dry mixture to the wet mixture, stirring gently until a soft dough forms.

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

Roll the chilled dough out to about 1/2-inch thickness and cut out cookies with a fluted cutter.

Brush the tops with beaten egg yolk and score with a fork for decoration.

Preheat the oven to 350°F (175°C) and bake for 15-18 minutes until golden brown.

Sprinkle with flaky sea salt immediately after removing from the oven.