Combine butter, sugar, and molasses in a large mixing bowl, mixing well; add eggs and vanilla, beating well.
Sift together flour, soda, salt, and mace in a small mixing bowl; stir into butter mixture, 1/2 cup at a time, mixing well after each addition. Stir in pecans.
Drop dough by heaping teaspoonfuls 2 inches apart onto greased and floured cookie sheets. Bake at 350° for 8 minutes. Remove from cookie sheets, and cool on wire racks.