Whisk flour, cocoa powder, baking soda, and salt in a medium bowl.
In a stand mixer, cream butter and sugar. Then add eggs and vanilla and mix just until combined. Gradually add the milk.
Mix the flour mixture into the wet ingredients just until combined.
Line a baking pan with parchment paper and using a 1-oz cookie scoop transfer the batter on the parchment paper.
Beat butter, powdered sugar, vanilla, and milk until creamy. Pipe the buttercream on one of the pies and sandwich the other one on top!