Combine butter, margarine, and sugar; beat with an electric mixer at medium speed until creamy. Add egg and vanilla extract; beat well.
In a separate bowl, combine flour, baking powder, and salt. Gradually add to creamed mixture, beating until blended.
Fill cookie press with dough, following manufacturer’s directions. Do not chill dough. For rolled cookies, chill dough 1 hour and roll to ⅛-inch thickness. Cut into desired shapes.
Bake at 350 degrees F 8 to 12 minutes, or just until edges begin to brown. Transfer to wire racks to cool. Decorate as desired.