In a large bowl with a handheld blender, beat the softened butter and granulated sugar until creamy. About 1 minute. Add the vanilla extract and continue to beat, scraping down the sides of the bowl as needed.

Add the all-purpose flour and salt and continue to beat slowly until a nice and thick cookie dough has formed. Lastly, add the egg yolk and continue to mix until just incorporated.

Line a baking sheet with parchment paper. Scoop and roll the dough into balls and place onto the baking sheet. Continue to do that until all of the cookie dough is used.

Press a thumb indentation into each of the cookies. The dough could crack slightly, but if it does, try to smooth it out as best as you can.

Chill in the freezer for 30 minutes or the fridge for 1 hour. The cookies will be hard and perfect for filling with jam.

Preheat the oven to 375℉/190℃/Gas Mark 5.

Line your baking sheets with parchment paper and place the cookies onto the sheets about 2-3 inches apart. Fill each cookie with about 1/2 tsp. of jam.

Bake for 13-16 minutes or until the edges of the cookies are just turning brown. Do not overbake! Remove from the oven and let cool for 5 minutes before transferring to a cooling rack.