Beat butter and sugar until creamed. Mix in egg and vanilla.
Mix in baking soda and spices.
Add flour, 1 cup at a time, until completely mixed.
Dump dough out onto an ungreased baking sheet. 13" x 18")
Sprinkle dough with a handful of flour to keep your hands from sticking as you press the dough into the pan. Continue to press until entire pan is evenly covered.
Bake at 350 degrees for 15 minutes.
Allow to cool completely.
While cookie bars cool, make frosting.
Mix butter, vanilla, milk and powdered sugar with an electric mixer until smooth and creamy. Spread over cooled cookie bars. Add sprinkles.