For the Cookies, preheat oven to 350°F (180°C). Mix flour, cocoa powder, baking soda, salt and garam masala in medium bowl; set aside. Mix buttermilk and vanilla in small bowl.
Beat butter and brown sugar in large bowl with electric mixer on medium-high speed until light and fluffy. Add egg; mix well. Add flour mixture alternately with buttermilk mixture, beating on low speed after each addition until smooth and scraping down sides of bowl occasionally. Spoon 1 tbsp (15 mL) of batter, 2 inches (5 cm) apart, onto parchment paper-lined large baking sheets (Cookies will spread so avoid crowding them on baking sheet).
Bake 8 minutes or until cookies are puffed and spring back when touched, turning baking sheets halfway through baking. Cool on baking sheets for 1 minute. Remove to wire racks; cool completely.
For the Filling, beat butter, icing sugar, marshmallow fluff, vanilla and cardamom in medium bowl with electric mixer on medium speed until light and fluffy.
To assemble the Whoopie Pies, place 1 tbsp (15 mL) filling on flat side of 1 cookie. Top with a second cookie, pressing gently to spread the filling. Repeat with remaining cookies. Roll edge of cookies in chopped pistachios. Store whoopie pies between layers of wax paper in airtight container in refrigerator up to 5 days.
