Preheat oven to 375F and prepare cookie sheets by lining with parchment paper. Set aside.
Beat butter in the bowl of a stand mixer (or using a hand mixer) until well-creamed.
Add sugars and beat on high speed until well-combined (about 1 minute).
Add eggs, one at a time, beating well after each addition.
Stir in vanilla extract.
In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
Set mixer to low speed and gradually stir dry ingredients into butter mixture. Stir until well-combined.
Stir in chocolate chips.
Drop cookie dough by rounded 2-tsp sized scoop onto prepared cookie sheets, at least 2" apart.
Bake on 375F for 10-12 minutes, or until cookies are beginning to turn golden brown.
Allow to cool completely on baking sheet before transferring to cooling rack or filling with buttercream (cookies will be flat and will be fragile when warm).
While cookies cool, prepare your frosting.
Chocolate Buttercream Frosting
In a microwave-safe bowl, microwave chocolate chips at 15-second intervals, stirring between intervals, until chips are completely melted. Set aside and allow to cool slightly (about 10 minutes, stirring occasionally).
Cream room temperature butter with an electric mixer. Stir in slightly cooled chocolate and beat well.
Gradually add sugar, scraping down the sides and bottom of bowl occasionally.
Sprinkle in salt and vanilla extract, stir well.
Gradually add heavy cream, increase speed to high and beat for 1 minute, pausing halfway through to scrape down sides of the bowl.
Spread or pipe chocolate frosting onto the bottom of one completely cooled cookie. Sandwich with another cooled cookie and repeat until all cookies have been filled.