Whisk together everything but the butter, then use your hands to work the butter into the mixture until you have crumbs. Set aside while you make the dough.
For the cookies:
Heat oven to 350 and line baking sheets with parchment paper. In a stand mixer, cream the butter, sugars and vanilla on medium speed until light and fluffy. Add in the egg and mix again.
In a bowl, whisk the flour, baking powder, cornstarch, baking soda and salt. Add to the mixer and mix on low until almost combined. Add the blueberries and fold them in.
Using a butter knife, dollop the jam onto the dough and swirl it through. You don't want to fully mix it in, you want to see some swirls.
Scoop out balls of dough using a medium cookie scoop. Flatten slightly and top with a little streusel. Bake for 8-10 minutes, until lightly browned on the edges and cooked through. Don't over bake.