1/2 cup (125 mL) unsalted butter, softened
1 cup (250 mL) granulated sugar
1 tsp (5 mL) vanilla
1-3/4 cups (425 mL) all-purpose flour
2/3 cup (150 mL) cocoa powder
1-1/2 tsp (7 mL) baking soda
1/2 tsp (2 mL) salt
1 cup (250 mL) buttermilk
In large bowl, beat butter with sugar until light and fluffy, about 2 minutes. Beat in egg; beat in vanilla.
In separate bowl, whisk together flour, cocoa, baking soda and salt; stir into butter mixture alternately with buttermilk, making 3 additions of flour mixture and 2 of buttermilk.
Drop by heaping 2 tbsp (30 mL), about 3 inches (8 cm) apart, onto parchment paper-lined baking sheets to make 24 mounds.
Bake in 350°F (180°C) oven until tops slightly crack and spring back when lightly touched, 12 to 14 minutes. Let cool on pan on rack.
1 ½ cups marshmallow fluff
1 cup butter
1 cup icing sugar
1 tablespoon vanilla extract
Cream together the butter, sugar and vanilla on medium speed until light and fluffy. Beat in marshmallow fluff until smooth.
Sandwich two chocolate cookies with a spoonful of marshmallow filling. Press together lightly.