Cream butter, brown sugar, and white sugar until light and fluffy. Add egg and vanilla, mixing until smooth. In another bowl, combine flour, cocoa powder, baking soda, and salt. Gradually add dry ingredients to the butter mixture until a soft dough forms. Roll the dough into small balls and place on a lined baking tray. Use your thumb or the back of a teaspoon to make an indentation in the center of each cookie. Chill the shaped cookies for about 20 minutes to prevent spreading.
Bake at 175°C (350°F) for about 10–12 minutes or until set. Let the cookies cool completely. Melt chocolate and cream together until smooth and glossy, then spoon into the center of each cookie. Add colorful sprinkles while the chocolate is still warm, then let it set before serving.
