1. Prepare the Cranberry Curd

1. Cook the Cranberries: In a medium saucepan, combine the cranberries and sugar over medium heat. Cook until the cranberries burst and release their juices, about 5-7 minutes. The mixture should become thick and syrupy.
2. Strain the Mixture: Remove from heat and strain the mixture through a fine mesh sieve to remove the skins. This will give you a smooth curd. Return the strained juice to the saucepan.
3. Whisk the Eggs: In a bowl, whisk together the eggs and lemon juice. Gradually add this mixture to the cranberry juice in the saucepan, stirring constantly to prevent the eggs from scrambling.
4. Thicken the Curd: Add the butter pieces and return to low heat. Stir continuously until the mixture thickens and coats the back of a spoon, about 8-10 minutes. Remove from heat and let cool. This curd can be made ahead of time and stored in the refrigerator until ready to use.

2. Make the Cookie Dough

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. Cream the Butter and Sugar: In a large mixing bowl, cream together the softened butter and sugar until light and fluffy, about 3-4 minutes. This step is crucial for achieving a light texture.
3. Add Egg Yolk and Vanilla: Add the egg yolk and vanilla extract, mixing until fully combined.
4. Combine Dry Ingredients: Gradually add the flour and salt, mixing just until the dough comes together. Be careful not to overmix, as this can lead to tough cookies.

3. Shape the Cookies

1. Form the Dough Balls: Using a cookie scoop or your hands, form the dough into 1-inch balls and place them on the prepared baking sheet, spacing them about 2 inches apart.
2. Create Indentations: Using your thumb (or the back of a spoon), gently press down in the center of each dough ball to create an indentation. This is where the cranberry curd will go, so make sure it’s deep enough!

4. Bake the Cookies

1. Bake: Bake in the preheated oven for 12-15 minutes, or until the edges are lightly golden. The centers will still look soft, but don’t worry; they will firm up as they cool.
2. Cool on the Baking Sheet: Remove from the oven and allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely. This step helps maintain their shape.

5. Fill with Cranberry Curd

Once the cookies are completely cool, spoon or pipe the cranberry curd into the center of each thumbprint. Don’t be shy—fill them generously! The curd will set as it cools, creating a delicious contrast with the buttery cookie.