Preheat your oven to 350 degrees F.
Line 2 cookie sheets with parchment paper.
In a large bowl sift together the flour, baking soda, unsweetened cocoa, and salt. Set aside.
In your stand mixer combine the butter and sugar, whisk until fluffy and pale. Beat in the egg and vanilla extract. Beat in half the flour mixture. Add the buttermilk and beat again. Beat in the rest of the flour mixture until everything is well combined. Stir in the chocolate.
Use a teaspoon and scoop small balls of batter on the cookie sheets, leaving a gap for a bit of spread. You should be able to get about 36 bits.
At this point I highly recommend popping the dough in the freezer for at least 30 minutes, but preferably 60 minutes. If you don't do this the cookies will spread too much and be too flat. You're call though. If you like flat sandwich cookies knock yourself out.
Remove the cookie sheets from the freezer and pop them in the oven. Bake for about 8 to 10 minutes. Remove and allow to cool for about 5 minutes. Then remove them from the cookie sheet and let them cool on wire racks.