To make the streusel place all ingredients in a bowl and mix with a fork until crumbly. Set aside.

Preheat the oven to 350 degrees.

In a large bowl cream together the butter, sugar, and brown sugar. Add in the egg and vanilla and mix well.

In a medium bowl combine the flour, cornstarch, baking powder, baking soda, and salt. Add the blueberries and toss to coat them.

Add the flour mixture to the butter mixture and gently combine.

Using a 1/4 cup cookie scoop, scoop batter onto a cookie sheet 3 inches apart.

Place a small amount of jam on top of each cookie and swirl. Top with the streusel.

Bake for 13-15 minutes or until the center of the cookies are just set.

Remove from oven and cool for 5 minutes before removing to a wire rack to cool completely.