Preheat the oven to 350˚F and line a rimmed baking sheet with parchment paper. Combine the butter and brown sugar in the bowl of a stand mixer fitted with the paddle attachment. Cream at medium speed for 2 to 3 minutes, or until light and fluffy.

Scrape down the sides of the bowl and add egg yolk and vanilla extract. Mix at medium speed for 30 seconds, or until incorporated.

Add the flour and salt and process at low speed, just until no streaks of flour remain.

Using a 1 tablespoon-sized cookie scoop, scoop the dough into even portions, then roll each into an even ball. Place on the prepared sheet pan, leaving about 2 inches between each cookie.

Using your thumb, or the back of a measuring spoon, gently press an indent into the cookie. Press down just enough to make an indent, without pressing all the way to the bottom of the cookie. If using jam, spoon about 1 teaspoon into the indent, then bake for 10 to 12 minutes, or until lightly golden brown. If using nutella, make the indents, but fill with 1 teaspoon of nutella after the cookies have been baked.

Set on a baking rack until completely cool.