Preheat oven to 350°. Cream butter, then gradually add the sugar, beating well. Mix the flour, salt and baking powder and add the first mixture, combining thoroughly. Roll out dough with a rolling pin until it's 1/4 inch thick, then cut into rectangles or any other shape you like. Put on ungreased cookie sheets, prick each with fork and bake for 20 to 25 minutes, or until lightly brown around edges.