Step 1

Preheat the oven to 350 degrees F. Line several large, rimmed baking sheets with parchment paper.

Step 2

For the cookies, in a medium bowl, whisk together the flour, cocoa, baking soda, baking powder, cream of tartar, and salt.

Step 3

In a large bowl with a handheld electric mixer or in the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, granulated sugar and powdered sugar until light and creamy, 2-3 minutes. Add the oil and eggs and mix until well-combined.

Step 4

Add the dry ingredients and mix until the dough comes together. It should be stiff, but not dry (and definitely not overly wet or sticky).

Step 5

Form the dough into balls about the size of a golf ball or thereabouts. I use my #20 cookie scoop (about 3 tablespoons of dough per ball). You can definitely make them smaller, too, if you'd like.

Step 6

Place the cookies several inches apart on the prepared baking sheet(s).

Step 7

Place 1/4 cup or so of granulated sugar in a shallow dish. Dip a flat-bottomed glass into the sugar (it helps to rub the bottom of the glass lightly with butter or oil before the first dip so the sugar adheres) and then press the glass onto a cookie, flattening to about 1/4-inch or slightly thicker. Repeat that process for all of the cookie dough balls.

Step 8

Bake for 8-9 minutes. The cookies will still be very soft in the middle but set around the edges. Don't overbake! Let them cool for a few minutes on the pan before transferring to a cooling rack to cool completely.

Step 9

For the frosting, with an electric mixer (handheld or stand mixer), cream together the butter and 1 cup of the powdered sugar until very light and fluffy, 1-2 minutes. Add the remaining powered sugar, sifted cocoa powder, heavy cream, and vanilla. Mix for 2-3 minutes until very thick and creamy.

Step 10

Spread the frosting evenly on the cookies. Decorate with sprinkles, if desired. Store well-covered in the refrigerator for 3-4 days.