Preheat oven to 350 degrees. Prepare two large baking sheets by adding silicone mats or parchment paper. Set aside.
Add flour, cocoa, baking soda, and salt to a medium bowl and whisk to combine. Set aside.
Add butter and sugar to a bowl fitted for a stand mixer. Beat until light and fluffy - about 3 minutes. Add egg and beat until combined. Scrape down sides of the bowl.
Add buttermilk to a measuring cup with a spout (or something similar). Add vanilla and whisk together.
Add 1/3 of flour mixture to butter mixture and combined. Add 1/2 buttermilk mixture and combine. Repeat with next 1/3 of flour mixture and then with last 1/2 of buttermilk mixture. Finished with final 1/3 of flour mixture.
Scoop whoopie pies onto prepared baking sheets using a small cookie scoop (1.5 tablespoons) and place them 2 inches apart. Bake whoopie pies until firm to the touch and edges are firm, about 9-11 minutes.
Remove from oven and let them cool for 10 minutes on baking sheet. Remove from baking sheet and let them cool completely before adding ice cream. At this point, remove ice cream from freezer to let it thaw a little.
Once cookies are completely cooled and ice cream is slightly thawed, scoop 1/3 cup of ice cream onto one half of cookie. Sandwich with another half and then wrap with plastic wrap. Repeat with rest of whoopie pies. Freeze pies until solid (about 1 hour) or you can enjoy them immediately.