preheat over to 375. position rack to center. line two baking sheets with parchment paper

sift flour, cocoa powder, baking soda and salt on waxed paper. in a large a bowl beat butter, shortening, and brown sugar on low until just combined. increase.speed to medium and beat for 3 minutes. add egg and vanilla and beat 2 more minutes.

add half of flour mixture and half of milk to batter, beat briefly on low. beat in remaining flour mixture and milk.

using a spoon, drop batter 1 tbsp at a time onto one of the baking sheets, spacing at least 2 inches apart. bake one sheet pan at a time got about 10 minutes or until pies spring back when pressed gently. remove from oven, let pies cool on sheet for 5 minutes then transfer to a rack to finish cooling.

to make filling: beat fluff and shortening for 3 minutes on medium. reduce to low, and add sugar and vanilla. beat until incorporated. increase speed to medium, beat until fluffy.

sandwich filling between the flat sides of two pies