Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats to help the cookies bake evenly and prevent the jam from sticking.

In a large mixing bowl or stand mixer fitted with the paddle attachment, beat the softened butter and 2/3 cup granulated sugar on medium speed for 2–3 minutes, until light and fluffy. Scrape down the bowl as needed so everything is fully combined.

Add the egg yolk, vanilla extract, and salt to the creamed mixture. Beat on low–medium speed until the mixture looks creamy and uniform.

Add the flour in two additions, mixing on low speed just until the dough comes together. It may look slightly crumbly at first, but should form a soft dough that holds when you squeeze it. Do not overmix.

Cover the bowl and chill the dough in the refrigerator for 20–30 minutes. This helps the cookies hold their shape and keeps the jam from spreading too much while baking.

Pour a few tablespoons of granulated sugar into a small bowl. Using a small cookie scoop or tablespoon, portion the dough and roll it between your palms into 1-inch balls (about 15 g each). Roll each ball lightly in sugar and place on the prepared baking sheets, spacing about 2 inches apart.

Using your thumb, the back of a small spoon, or the handle of a wooden spoon, gently press into the center of each dough ball to create a deep indent without pressing all the way through. If the edges crack, gently press them back together.

In a small bowl, stir the jam until smooth. If it is very thick, add 1–2 teaspoons of water and stir until the jam is loosened and spoonable.

Using a small spoon or piping bag, fill each indent with about 1/2 teaspoon of jam. Do not overfill, as the jam will bubble slightly as the cookies bake.

Bake one sheet at a time for 12–14 minutes, or until the edges are just set and barely golden while the centers remain soft. If any indent puffs up during baking, gently press it back down with the back of a spoon while the cookies are still hot (use caution; the jam will be very hot).

Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely. The jam will thicken as it cools and the cookies will firm up while staying tender. Serve at room temperature or pack into tins for gifting.