Cream the softened unsalted French butter and granulated sugar in a mixing bowl on medium speed for 4-5 minutes until pale and fluffy.

Add a large egg and vanilla extract, mixing on low speed until just combined and smooth.

In a separate bowl, whisk together flour, baking powder, and sea salt; gradually add to the wet mixture, folding gently.

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

Preheat the oven to 350°F (175°C) and roll the chilled dough to 1/2-inch thickness, cutting out cookies.

Brush the cookie tops with beaten egg yolk and lightly score with a fork.

Bake for 15-18 minutes until golden around the edges and soft in the center.

Sprinkle with flaky sea salt while warm, and allow to cool on racks before serving.