In a large bowl combine the sugar, egg, molasses, and canola oil.
In medium bowl sift together 3 ½ cups flour, salt, baking powder, soda, ginger, and cinnamon.
Add the dry ingredients to the wet ingredients alternately with buttermilk. You may need to add remaining flour to stiffen the dough if needed. Chill for 2 hours.
Remove the cookie dough onto a floured surface and cut into thirds. Place the dough you are not rolling back in fridge to keep chilled. Lightly coat dough with flour and shape into a round ball. Roll to about ½" thick. Cut with round cookie cutter. Repeat with remaining dough.
Bake for 10 minutes or until cookie looks dry on top and a slight dent remains when touching top of cookie with finger. Cool completely on a wire rack.
For the icing
In a large saucepan, melt the butter and mix in with the confectioners' sugar along with vanilla and 2 tablespoons milk. Add more milk as needed to make spreading consistency.
Frost completely cooled cookies and store in an airtight container.