Combine butter and brown sugar in a large mixing bowl, mixing well. Add egg and vanilla, beating well.
Sift together flour, soda, and salt in a small mixing bowl. Gradually add to creamed mixture, stirring well. Stir in pecans. Spread mixture in a greased 9-inch square baking pan. Bake at 350° for 25 minutes. Cool and cut into 1 1/2-inch squares.