Cake: Beat together sugar and eggs, until blended. Heat milk and butter until butter is melted. Do not boil. Add all at once to egg mixture.
Combine flour and baking soda; gradually stir into mixture, stirring until well blended. Fold in blueberries. Spread evenly into a greased 13 x 9 inch pan.
Topping: In a bowl, combine brown sugar and cinnamon. Cut in butter until crubmly, and sprinkle over cake. Bake in a 350 degree oven for 45 minutes or until a toothpick inserted in center comes out dry.
Sauce: In a saucepan combine brown sugar, flour and salt. Add boiling water and cook until slightly thickened. Remove from heat and stir in butter and vanilla. Serve warm over individual servings of blueberry cake. Garnish with a dab of whipped cream or whipped topping.