Preheat oven to 325°F and line a baking sheet with parchment.
Sift flour, salt, cocoa powder, and baking soda; set aside.
In a stand mixer, beat sugar, egg, and vanilla until creamy.
Melt butter and chocolate chips separately; combine them.
With mixer running, pour chocolate-butter mixture into the bowl.
Slowly add dry ingredients until a sticky dough forms.
Chill dough in the refrigerator for 1 hour.
Divide dough into two equal parts and shape into 2-inch-wide logs.
Wrap logs in plastic wrap and chill 2 hours or overnight.
Slice logs into ¼-inch rounds (rest dough 10 minutes if very firm).
Cut into cleaner circles using a 2-inch cookie cutter (optional).
Place slices on baking sheet and bake 20 minutes.
Cool 5 minutes, then transfer to a wire rack.
Mix all filling ingredients until smooth and creamy.
Spread 1 tsp filling onto one cookie and top with a second cookie.
Gently press to spread filling evenly; repeat with remaining cookies.