1. In a pan, add cranberries, 1/2 cup sugar, and 1/3 cup water. Cook until it reaches thick jam consistency and cranberries are soft. Take it off the heat and cool. Blend it to a pulse to make the sauce. Set aside.
2. In a pan, add prunes, 1/2 cup sugar, and 1/3 cup water. Cook until it reaches thick jam consistency and prunes are soft. Take it off the heat and cool. Blend it to a pulse to make the sauce. Set aside.
3. Make the cookies: Using a handheld or stand mixer with a paddle attachment beat the butter on high speed until creamy, about 1 minute. Switch mixer to medium speed and add the sugar and vanilla.
4. Next add the egg yolk and beat for a minute. Add the flour and mix nicely to form a soft dough. Do not knead the dough. Press the dough down to compact it and tightly cover with plastic wrap to chill until firm, at least 1 - 2 hours.
5. Preheat oven to 350°F (177°C). Line two baking sheets with parchment paper. Shape the cookie dough into balls of equal size approximately 1 tablespoon of dough per ball.
6. Make an indentation with your thumb into each ball. The dough may crack slightly when you press your thumb into it. Simply smooth it out with your fingers if you can. Otherwise, it's perfectly nice to have a few cracks.
7. Fill one cookie with 1/2 teaspoon of cranberry sauce while the other one with the prune sauce.
8. Bake the shortbread thumbprint cookies for 14 -15 minutes, or until very lightly browned on the edges.
9. The cookies will puff up and spread slightly. Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.

Enjoy your meal!