Preheat oven to 350 degrees. 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.
2 ½ cups (313 g) all-purpose flour, spooned and leveled, ½ teaspoon baking powder, ½ teaspoon salt
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.)
1 ¼ cups (250 g) granulated white sugar, 1 cup (224 g) unsalted butter, softened
Add in the egg and vanilla and mix on medium speed until pale in color and fluffy, 1-2 minutes.
1 egg, at room temperature, 1 tablespoon vanilla
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 tablespoon cookie scoop.
Place the dough on the prepared cookie sheet, then sprinkle with sugar. (Bake 6 cookies at a time.)
¼ cup (50 g) granulated sugar for sprinkling over the cookies (optional)
Bake the cookies for 10 minutes, then let them cool on the baking sheet for 5 minutes. Transfer the baked cookies to a cooling rack to finish cooling.