PREHEAT oven to 350 degrees F. GREASE and FLOUR a 9x13 inch baking dish.
In a large bowl, STIR together the flour and sugar. CUT in butter using a pastry blender or a fork until pea-sized. REMOVE 3/4 cup of the mixture and MIX cinnamon into this mixture. SET ASIDE.
ADD baking powder and salt into the remaining mixture. ADD egg yolks, milk and vanilla extract. Using an electric mixer, MIX on low speed for 3 minutes.
In a large glass or metal bowl, WHIP egg whites just until stiff peaks begin to form. Carefully FOLD egg whites into the batter.
SPREAD batter evenly into the prepared 9 x 13 pan. SPRINKLE blueberries over the batter. SPRINKLE the reserved crumb mixture evenly over the top.
BAKE for about 40-50 minutes or until the cake springs back when touched lightly in the center. (Start checking for doneness at 35 minutes).
NOTE: I often use a food processor to quickly pulse the flour and sugar and then to cut in the butter. I quickly pulse until butter is pea-sized. I then transfer mixture to a standing mixer bowl. More dishes to wash but less hassle getting the butter to break up into pieces.
You can also use a square baking pan for this recipe. BAKE for about 50 minues minutes or until the cake springs back when touched lightly in the center. (Start checking for doneness at 50 minutes).