Preparation Preheat oven to 350°F. Lightly butter 8-inch square baking pan. Mix flour, brown sugar, baking powder and salt in medium bowl. Whisk butter, egg and vanilla in large bowl to blend. Add flour mixture. Stir to blend. Mix in nuts. Press dough over bottom of prepared pan. Bake blondie until golden and tester inserted into center comes out clean, about 25 minutes. Transfer pan to rack and cool. Bring cream to simmer in heavy small saucepan. Remove from heat. Add chocolate. Stir until chocolate melts and mixture is smooth. (Can be made 1 day ahead. Store blondie airtight at room temperature. Cover and refrigerate chocolate sauce. Rewarm sauce over medium-low heat before using.) Cut blondie into 16 squares. Place 2 squares in each of 8 large sundae dishes. Thread marshmallows onto 4 skewers; hold over gas flame or under broiler until soft and golden. Place 2 scoops ice cream atop blondies in each dish. Spoon sauce over. Top with 2 marshmallows.
Preparation Preheat oven to 350°F. Lightly butter 8-inch square baking pan. Mix flour, brown sugar, baking powder and salt in medium bowl. Whisk butter, egg and vanilla in large bowl to blend. Add flour mixture. Stir to blend. Mix in nuts. Press dough over bottom of prepared pan. Bake blondie until golden and tester inserted into center comes out clean, about 25 minutes. Transfer pan to rack and cool. Bring cream to simmer in heavy small saucepan. Remove from heat. Add chocolate. Stir until chocolate melts and mixture is smooth. (Can be made 1 day ahead. Store blondie airtight at room temperature. Cover and refrigerate chocolate sauce. Rewarm sauce over medium-low heat before using.) Cut blondie into 16 squares. Place 2 squares in each of 8 large sundae dishes. Thread marshmallows onto 4 skewers; hold over gas flame or under broiler until soft and golden. Place 2 scoops ice cream atop blondies in each dish. Spoon sauce over. Top with 2 marshmallows.