Preheat oven to 375°F. Prepare an 8x8 inch pan by spraying it well with non-stick cooking spray and lining it with parchment paper so that the parchment comes up the side and hangs over the edge.
In a large bowl, whisk together the flour, sugar and baking powder for the bottom crust. Add the butter and egg yolk and cut it into the flour mixture with a pastry cutter or fork until the mixture resembles a wet sand.
With floured fingers, press the bottom crust evenly into the bottom of the pan.
Gently spread the blueberry jam evenly on top of the crust.
Combine all of the topping ingredients into a bowl. With your fingers, mix them together, being sure to work the butter into the mixture. When it is mostly uniform and can form a clump in your palm, sprinkle it on top of the jam.
Place pan into preheated oven and bake for 25-30 minutes, or until the jam has begun to bubble through and the crisp topping has browned.
Remove from oven and allow to cool completely before cutting.