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 1/2 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.

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.