Blueberry Coffee Cake

This blueberry coffee cake rocked my world a few moths ago... and I got the recipe!! It's so good! We've been nibbling on it all day! 2 cups whole blueberries
1/2 cup water
1 T lemon juice
1 cup sugar
1/3 cup cornstarch
3 cups flour
1 cup sugar
1 T baking powder
1 tsp salt
1 tsp ground cinnamon
1/4 tsp ground mace
1 cup butter
2 slightly beaten eggs
1 cup milk
1 tsp vanilla
1/2 cup sugar
1/2 cup flour
1/4 cup butter
1/2 cup chopped nuts (optional)

In a saucepan combine fruit and the water.  Simmer, covered, about 5 minutes or till fruit is tender.  Stir in lemon juice.  Mix the 1 cup sugar and cornstarch; stir into fruit mixture.  Cook and stir till thickened and bubbly. Cool.
In a mixing bowl stir together the 3 cups flour, 1 cup sugar, baking powder, salt, cinnamon, and mace.  Cut in the 1 cup butter till mixture resembles fine crumbs.  Combine eggs, milk, and vanilla.  Add to flour mixture, mixing till blended.  Spread half of the batter in a greased 13x9 pan.
Spread the cooled fruit mixture over the batter.  Spoon the remaining batter in small mounds over the fruit mixture, spreading out as much as possible. Combine the 1/2 cup sugar and 1/2 cup flour; cut in the 1/4 cup butter till mixture resembles coarse crumbs.  Stir in nuts.(optional) Sprinkle nut mixture over batter in pan.  Bake in a 350 F oven 45-50 minutes.  Cool.