The best butter cookies

2 cups (4 sticks) butter; softened
1-1/2 cups sugar
2 eggs
1 teaspoon vanilla extract
5 cups all-purpose flour
2 teaspoons baking powder
1/2 teaspoon salt
Buttercream icing:
1/2 cup (1 stick) butter;softened
1/8 teaspoon salt
2 cups confectioners` sugar
2 tablespoons cream or milk

Preheat oven to 375 F. Cream butter and sugar in large mixing bowl until light and fluffy. Beat in eggs andvanilla. Combine flour, baking powder and salt. Gradually add to butter mixture; mix well. Divide dough in fourths. Roll out one-fourth of dough at a time on lightly floured surface to 1/8-inch thickness. Keep remaining dough refrigerated. Cut into desired shapes with floured cookie cutters. Place 1 inch apart on unbuttered cookie sheets. Bake 6 to 8 minutes or until lightly browned. Transfer cookies to wire racks; cool completely. Decorate as desired. Store in cool place in airtight metalcontainers up to 1 month. Freeze up to 3 months.
Buttercream icing:
Cream butter and salt together; add sugar and stir briskly. Stir in cream or milk and beat well. Add additional sugar or milk if necessary for spreading consistency.