Preheat the oven to 350 degrees F and line baking sheets with parchment paper.

Cream the butter, granulated sugar, and powdered sugar in a large bowl with an electric mixer until pale and fluffy (3 to 5 minutes on medium-high speed).

Stir in the melted chocolate until incorporated.

Scrape the bottom and sides of the mixing bowl with a silicone spatula, then add the oil, mixing on medium speed until smooth.

Add the eggs, one at a time, mixing on medium speed until incorporated.

Scrape the bottom and sides of the mixing bowl with a silicone spatula, then add 4 ½ cups of the flour, along with the cocoa, salt, baking soda, and cream of tartar, and mix until combined.

If the dough seems sticky, add the remaining flour, 2 tablespoons at a time, until it can be handled easily.

Scoop 2 tablespoon-sized balls of dough, and place them on the prepared baking sheets.

Dip the bottom of a small drinking glass into granulated sugar, then use it to press the cookie dough balls flat. (Allow some of the dough to extend beyond the glass, creating a ruffled edge.)

Bake the cookies for 7 to 9 minutes, or until set around the edges but still very soft and tender.

Cool completely, then frost with buttercream and garnish with chopped peppermint bark.

Place the sour cream and butter in a medium mixing bowl and whip on medium-high speed until smooth.

Add the powdered sugar, a cup at a time, stopping to scrape the bottom and sides of the bowl with a silicone spatula after each addition.

Stir in the melted chocolate, cocoa powder, salt, and peppermint extract, then turn the mixer up to medium-high speed and whip until smooth and fluffy.

Thin the frosting as needed with milk.

Spread about 1 ½ tablespoons of frosting on each cooled cookie, then top with a pinch of chopped peppermint bark, for garnish.