Begin by placing 1 cup of softened unsalted French butter and 1 cup of granulated sugar into your mixing bowl. Using a hand mixer or stand mixer, beat them together on medium speed for about 3–5 minutes until the mixture becomes pale and fluffy.

Add in 1 whole egg and 1 teaspoon of vanilla extract to the creamed mixture. Continue mixing on low speed until the ingredients are fully combined and smooth, roughly 1–2 minutes.

In a separate bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon of baking powder, and 1 teaspoon of sea salt. Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Turn the dough out onto a lightly floured surface and gently knead it a few times to ensure it’s uniform. Shape it into a log, about 2 inches in diameter. Wrap the dough tightly in plastic wrap and refrigerate for at least 1 hour.

After chilling, preheat your oven to 350°F (175°C) and line a baking tray with parchment paper. Unwrap the chilled dough and slice it into rounds about 1/2 inch thick.

Brush the tops of the cookies with a beaten egg yolk. Using a fork, gently score a crisscross pattern on each cookie.

Bake for 15–18 minutes until the edges are golden brown, and the center is just set.

Sprinkle with flaky sea salt while still warm and allow to cool on the tray for a few minutes before transferring to a wire rack.