Preheat oven to 350 degrees F. Lightly grease and flour three 3" x 5" mini loaf pans.
In a medium bowl combine 1 cup plus 2 teaspoons of the cake flour with the cornstarch and baking powder.
In a medium bowl, using and electric mixer beat 1 cup sugar and 1 cup plus 2 table spoons of the butter, 1/2 teaspoon vanilla, and the lemon extract for about 5 minutes or until light and creamy. Add the eggs one at a time, beating well after each addition and adding 2 to 3 tablespoons of the flour mixture with the last egg; the mixture may seem curdled. Beat for 3 or 4 minutes or until creamy again.
With a rubber spatula, fold in the remaining flour mixture until thoroughly incorporated. Stir in the chocolate chips.
In a small bowl, with a fork mix together the remaining 1/4 sugar, 2 tablespoons butter and 3/4 teaspoons vanilla. Add the remaining 1/2 cup flour and mix until crumbly.
Fill the prepared loaf pans with the crumb cake batter and sprinkle the crumb mixture over the top.
Bake the crumb cake for 35 to 40 minutes or until the middle of the cake springs back when touched lightly or a wooden toothpick inserted in the center comes out clean.