Combine 1/4 cup condensed milk, butter, and next 3 ingredients (butter through egg white) in a bowl (batter will be very stiff). Coat bottom of a 13 x 9-inch baking pan with cooking spray. Press two-thirds of batter into prepared pan using floured hands; pat evenly (layer will be thin).
Bake at 350° for 10 minutes. Combine 1/2 cup condensed milk and marshmallow creme in a bowl; stir in morsels. Spread marshmallow mixture evenly over brownie layer. Carefully drop remaining batter by spoonfuls over marshmallow mixture. Bake at 350° for 30 minutes. Cool completely in pan on a wire rack.