Add the blueberries, sugar, lemon juice, vanilla and cornstarch to a medium pot. Cook over medium-low heat for 30-40 minutes until the blueberries are very thick and measure out to ½ cup (120 ml). About 15 minutes in, smush some of the blueberries with the back of a wooden spoon. Remove from the heat and allow to completely cool.

Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper and set aside.

In a large bowl, whisk together the flour, baking powder and salt. Then set aside the flour mixture.

In a large mixing bowl cream together the softened butter and granulated sugar with an electric mixer on high speed until fluffy, about two minutes. (You can also use the bowl of a stand mixer fit with a paddle attachment.)

Add in the egg and vanilla and mix on medium speed until pale in color and fluffy, 1-2 minutes.

Slowly add in the dry ingredients to the wet ingredients. Mix on low speed just until the dough pulls together. Scrape the sides of the bowl with a spatula as necessary.

Fold in the cooled blueberry jam with a rubber spatula.

Scoop the cookie dough into balls with a large 2 tbsp cookie scoop.

Place the dough on the prepared cookie sheet, then sprinkle with sugar. (Bake 6 cookies at a time.)

Bake the cookies for 10-13 minutes, then let them cool on the baking sheet for 5 minutes. Transfer the baked cookies to a cooling rack to finish cooling.