Place the flour, cocoa powder, baking soda, cornstarch, and salt in a medium-sized bowl and stir together until combined, set aside.
In a large bowl whisk together the butter with the brown sugar until smooth.
Add in the egg and vanilla, stir until smooth.
Pour the dry ingredients into the wet and stir until combined. This will take a few minutes and will seem like the mixture doesn’t want to go together but it will. The dough will be thick.
On a parchment paper-lined sheet tray, place 1.5 tablespoon-sized dollops of the dough close together but not touching, I like to use a 1.5 tablespoon cookie scoop for this. Cover and place in the refrigerator for at least 2 hours, or overnight to chill.
Preheat the oven to 350 degrees F. Line sheet trays with parchment paper, set aside. Roll the dough into round balls, the dough will be very stiff at first but will soften slightly. Roll the dough in the granulated sugar and place them on the lined sheet trays 2 inches apart.
With the bottom of a flat object, I like to use a small drinking glass, press down the cookies to form a disc.
Bake for 8-10 minutes, until they are slightly puffed and no longer look raw. Let the cookies cool on the sheet tray for 10 minutes, then transfer to a wire rack to cool completely.
While the cookies cool, make the frosting by placing the powdered sugar, cocoa powder, and salt in a medium-sized bowl and stirring it together until combined.
Add the butter to the body of a stand mixer with the paddle attachment or into a large bowl with an electric hand mixer, whip the butter until smooth.
Add the powdered sugar mixture a little at a time until fully combined, the mixture will be thick.
Add the vanilla and 2 tablespoons of the heavy cream, stir to combine then whip on medium-high speed until light and fluffy, for 3 minutes. If the frosting looks a little thick, add the additional 1 tablespoon of heavy cream and whip for 1 additional minute.
When the cookies are cool, frost with the frosting, about 1-2 teaspoons per cookie. Immediately decorate with sprinkles if using, serve immediately.