Cream butter and sugar until smooth, then mix in egg and vanilla until fully combined.

Whisk flour, cocoa powder, and salt together, then add to the butter mixture and mix until a soft dough forms.

Cover and refrigerate the dough for 30–40 minutes.

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

Roll dough into tablespoon-sized balls and place 2 inches apart on the baking sheet.

Gently press a thumbprint into the center of each cookie.

Bake for 10–12 minutes until edges are set and centers are soft. Re-press centers gently if needed.

Cool slightly, then fill each center with your chosen filling. Allow filling to set before serving.