In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar together for 2 to 3 minutes until light and fluffy. (You can also use a hand mixer or mix by hand, but a stand mixer makes it so much easier.)

Add the eggs and vanilla extract and mix again until fully combined and smooth.

In a separate bowl, whisk together the flour, cocoa powder, salt, cornstarch, and oats.

Fold the dry ingredients into the wet ingredients until just combined.

Fold in 1 cup of mini chocolate chips until evenly distributed throughout the dough.

Scoop into 24 equal-sized balls, about 2 tablespoons each, and place them on the prepared baking sheets with space between each cookie.

Bake at 350°F for 11 to 13 minutes, until the edges are set but the centers still look slightly underdone.

Let the cookies cool completely on the baking sheet. This is important! Warm cookies will melt the filling.

While the cookies cool, make the filling. In the bowl of a stand mixer, beat the butter for 2 minutes until light and fluffy.

Add the cocoa powder and beat again until fully combined and smooth.

Add the vanilla extract and heavy cream and beat to combine.

Slowly add the powdered sugar, beating between additions, until you reach your desired consistency.

To assemble, flip half of the cooled cookies upside down. Spread or pipe a generous layer of chocolate buttercream onto each one.

Sprinkle a few mini chocolate chips over the buttercream on each cookie.

Top with the remaining cookies (right side up) and gently press to create a sandwich. Repeat with all cookies. Enjoy!