Preheat the oven to 350°F (175°C). Grease a muffin pan so the cups release easily after baking.
Mix the butter and sugar until smooth. Add the egg and stir until combined.
Add cocoa powder, flour, and salt. Mix until a thick chocolate dough forms. It should be soft but not sticky.
Divide the dough between the muffin cups. Press it into the bottom and up the sides to form small cups. The center should be hollow.
Bake until the cups look set and dry on top. The edges should feel firm when touched.
Let the cups cool completely in the pan so they keep their shape.
In a clean bowl, mix the cream cheese and powdered sugar until smooth and creamy.
Add a small amount of peppermint extract and mix again. The filling should be thick and smooth.
Spoon or pipe the filling into the cooled cookie cups, almost to the top.
Add a few drops of the red swirl on top. Use a toothpick to gently swirl it into the filling.
Sprinkle crushed peppermint candy in the center.
Chill the cups until the filling feels firm and holds its shape when cut.