In a large bowl, cream butter and sugars until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, baking soda and salt; gradually add to the creamed mixture and mix well. Stir in toffee bits (dough will be stiff). , Drop by rounded teaspoonfuls 2 in. apart onto ungreased baking sheets. Bake at 350° for 10 minutes or until firm (do not brown). , In a small bowl, beat the butter, sugar, vanilla and enough cream to achieve spreading consistency. Spread on the bottom of half of the cookies; top with remaining cookies.