Preheat the oven to 350°F (175°C).
In a mixing bowl, beat the softened butter and sugar until creamy.
Add in the egg and vanilla, and mix until well combined.
Sift together the flour, cocoa powder, and salt, then gradually add to the wet ingredients.
Fold in the chocolate chips yet this time.
Scoop tablespoon-sized portions of dough and roll into balls.
Place the dough balls on a lined baking sheet and press your thumb in the center of each.
Fill each indentation with chocolate chips or your desired filling.
Bake for 12–15 minutes, until the edges look firm but the centers are still soft.
Cool the cookies completely on a wire rack.