01 -
Beat the softened salted butter and granulated sugar on medium speed until smooth and slightly pale, about 2 minutes.
02 -
Beat in the egg yolks one at a time, then mix in the vanilla until the mixture looks glossy.
03 -
Whisk the flour and baking powder in a separate bowl. Add to the butter mixture and mix on low just until a soft dough forms.
04 -
Divide the dough into 2 discs, wrap tightly, and refrigerate for at least 1 hour, or until firm enough to roll.
05 -
Heat the oven to 325°F. Roll one dough disc to 1/4 inch thick on a lightly floured surface and cut into 2-inch rounds. Place on parchment-lined baking sheets.
06 -
Beat the egg with water. Brush a thin layer over the cookies, score the tops lightly with a fork, and sprinkle with a few flakes of sea salt if using. Chill the cut cookies for 10 minutes.
07 -
Bake for 16-18 minutes, rotating the pan once, until the edges are deep golden and the centers are set. Cool on the pan for 5 minutes, then move to a rack to cool completely.