In a large bowl, cream together the butter and sugars with an electric hand mixer until light and fluffy.
Beat in the egg, then add the melted chocolate chips and beat to incorporate.
Sift in the flour, cocoa powder, baking powder, and salt. Beat until just combined.
Scoop a rounded tablespoon of cookie dough into each cup of a mini muffin tin.
Use your fingers to spread the dough evenly across the bottoms and up the sides of the cups, being careful not to spill over the edges.
Bake for 8 minutes, until cookies have slightly puffed up. Let cool completely. If the centers have puffed up, use the back of a wooden spoon to reshape.
Prep the decorations: Add 4 ounces (100 grams) of chocolate chips to a liquid measuring cup or heatproof bowl. Pour over the warm heavy cream and let sit for 1 minute, then stir until well combined and smooth.
Pour the ganache into the cooled cookie cups until level with the tops.
Decorate the tops of each cookie cup with mini marshmallows and let sit at room temperature until the ganache has firmed up about 1 hours.
In a liquid measuring cup or heatproof bowl, melt the remaining chocolate in the microwave, then allow to sit at room temperature until thickened, about 10-15 minutes.
Use to scissors to cut the curved end of the mini candy canes into small handles.
Use a knife to gently lift the cookie cups from the muffin tin so one side sticks up. Dip the ends of the candy cane handles into the cooled melted chocolate, then stick onto the sides of the cookie cups. Let set for at least 1 hour before serving.