Chocolate Sandwich Cookies with Salted Caramel Russian Buttercream

For the cookies:
In a medium bowl, whisk flour, cocoa, baking soda, and salt; set aside until needed.

In the bowl of a stand mixer fitted with the flat beater or BeaterBlade attachment, cream butter and sugar on medium speed for about 2 minutes until it lightens in color. Add egg and vanilla, continue to mix until no traces of egg remain. Turn mixer speed down to low and add cocoa mixture. Combine until no traces of dry flour or cocoa remain, scraping bowl if needed. Wrap dough in plastic and chill for at least 1 hour.

Preheat oven to 350ºF. Place rack in center of oven and line 2 half sheet pans with silpat liners. Using a #50 scoop (about 1 tablespoon), roll level scoops of chilled cookie dough in the palm of your hand to form a ball. Coat with granulated sugar and space evenly on sheet pan, 12 per pan.

Bake for 9–10 minutes. Remove from oven and cool for 10 minutes before transferring to a cooling rack to cool completely.

For the filling:
In a medium mixing bowl, beat room temperature butter using a hand mixer for 2–3 minutes, until light in color and very fluffy. Add chilled dulce de leche and continue beating with a hand mixer for 1 minute.
Taste the buttercream carefully and add just enough salt to achieve the level of salted caramel flavor you desire, beating well so the salt is evenly distributed.

To assemble the cookies, place the filling in a disposable pastry bag and squeeze a quarter-size dollop on the flat side of one cookie, then add a second cookie, so flat sides are facing each other; squeeze gently to spread filling until it reaches the edges of the cookies.

Serve at room temperature, or better yet, chill for 10–15 minutes before serving.