1. Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

Make the Crumble: In a medium bowl, cream together the softened butter and sugar until light and fluffy. Add the flour and salt. Mix until the mixture resembles coarse crumbs. Set aside.

Make the Cookies: In a separate medium bowl, cream together the softened butter and sugar until light and fluffy. Beat in the egg yolk and vanilla extract until well combined.

1. In a small bowl, whisk together flour and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
2. Roll the dough into 1-inch balls. Place the balls onto the prepared baking sheets, leaving some space between each cookie.
3. Use your thumb or the back of a small spoon to make an indentation in the center of each cookie.
4. Fill each indentation with about 1/4 teaspoon of jam.
5. Sprinkle the crumble mixture evenly over the tops of the cookies.
6. Bake for 12-15 minutes, or until the edges are lightly golden brown.
7. Let the cookies cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.