Chocolate Snowswirl Fudge

3 cups (18 oz.) semi-sweet chocolate chips
1 can sweetened condensed milk
4 Tbsp. butter or margarine
1 1/2 tsp. vanilla extract
dash salt
1 C. chopped nuts
2 C. mini marshmallows

Melt chips with milk, 2 Tbsp. butter, vanilla and salt. Remove from heat; stir in nuts. Spread evenly into foil lined 8 or 9 inch square pan. Melt marshmallows with remaining butter. Spread on top of fudge. With knife, swirl through top of fudge.
Chill at least 2 hours or until firm. Turn fudge onto cutting board; peel off foil and cut into squares. Store loosely covered at room temperature.