Combine 1/4 cup condensed milk, butter, and next 3 ingredients in a large bowl (batter will be very stiff). Coat bottom of a 13 x 9-inch metal baking pan with cooking spray. Press two-thirds of dough into prepared pan using fingers coated with cooking spray; pat evenly (layer will be thin).
Bake at 350 for 10 minutes.
Combine remaining 1/2 cup condensed milk and marshmallow creme in a bowl; stir in morsels.
Spread marshmallow mixture evenly over brownie layer. Carefully drop remaining dough by spoonfuls over marshmallow mixture.
Bake at 350 for 25 minutes (a wooden pick will not test clean). Cool completely in pan on a wire rack.