Cream 1/4 of the butter and the brown sugar until fluffy. Beat in egg and molasses.
Mix flour with cinnamon, ginger, and baking soda. Add to creamed mixture alternately with the buttermilk. Pour batter into an 8 or 9 inch microwave safe glass pan.
Microwave for 4 to 5 minutes on medium. Rotate 4 times while cooking. Microwave 2 to 3 minutes on high. Cake is done when a toothpick inserted into the middle comes out clean. Serve warm or cold with Lemon Sauce.
To Make Lemon Sauce: Combine the white sugar and cornstarch. Stir until well mixed; add water. Microwave 2 to 3 minutes. Stir every 30 seconds, until mixture has thickened and boiled for 30 seconds. Watch carefully so that it does not boil over. Add the remaining 2 tablespoons butter, lemon zest, and lemon juice. Serve warm or cold.