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

In a bowl, whisk together all-purpose flour, unsweetened cocoa powder, baking soda, and salt.

In another bowl, cream together the unsalted butter, granulated sugar, and brown sugar using a hand mixer until light and fluffy.

Add the egg and vanilla extract to the butter-sugar mixture and blend until combined. Add purple food coloring if desired.

Gradually fold the flour mixture into the wet ingredients using a spatula, mixing until just combined.

Scoop about 2 tablespoons of dough, flatten into a disk, and place 1 teaspoon of blueberry jam in the center. Wrap dough around jam and seal edges.

Roll the filled dough balls in granulated sugar and place on prepared baking sheets, 2 inches apart.

Bake for 10-12 minutes until edges are set. Cool on pan for 5 minutes, then transfer to a wire rack.