Cream butter and sugars in mixer. Add egg and vanilla; mix through. Sift flour and cocoa together; add slowly to the creamed mixture until just incorporated. Scoop 2 Tbsp. batter per cookie onto greased cookie sheet; bake at 350 degrees for 8-10 minutes. Remove from oven; cool.
In small bowl, whip cream and powdered sugar until stiff peaks form. Place cookie in the center of plate. Layer with whipped cream and strawberry slices. Top with second cookie and a small amount of cream. Garnish with strawberry and mango.