Preheat the oven to 350°F. Cream the butter and sugar together until light and fluffy. Beat in the egg and vanilla. Whisk together the flour, cocoa powder, baking soda, salt, and cornstarch. Stir into the wet ingredients.
Roll the dough into 1-inch balls and bake for 9-11 minutes or until set. Do not overbake the cookies. They should like slightly doughy when they are taken out of the oven. Gently press ½ of a marshmallow on top of each cookie while they are still warm.
Let cool on the cookie sheet for a few minutes and then remove to a wire rack to cool completely.
Frosting Instructions
Melt the butter in a medium size glass bowl in the microwave, for about 30 seconds. Add the cocoa powder, corn syrup, powdered sugar, and milk. Stir to combine.
Microwave for about 20 seconds more. Stir again. Spoon the warm frosting over the cooled cookies and allow it to set.
Notes
The frosting will be softly set over the cookies in about 20 minutes. If you would like it to be firm, as pictured in the photos, allow it to set overnight.
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