Preheat Oven: Set oven to 350°F (175°C) and line a baking sheet with parchment paper.
Mix Dry Ingredients: In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
Cream Butter and Sugars: In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
Combine Wet Ingredients: Beat in the egg and vanilla extract until well combined.
Combine Mixtures: Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Add Color (Optional): If desired, add a few drops of purple food coloring to the dough and mix until evenly distributed.
Form Cookies: Scoop out tablespoons of dough, flatten slightly, and place a small dollop of blueberry jam in the center. Fold the dough around the jam to seal, forming a ball, and place on the prepared baking sheet.
Sprinkle Sugar: Sprinkle the tops with granulated sugar.
Bake: Bake for 10-12 minutes, until the edges are set but the centers are still soft.
Cool: Allow to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.