Make the streusel first by combining the flour, brown sugar, cold cubed butter, cinnamon and salt in a bowl. Use a fork or pastry cutter to cut the butter into the dry ingredients until pea-sized crumbs form. Refrigerate while making the cookie dough.
In a large bowl, beat the softened butter with the brown sugar and granulated sugar until light and fluffy, 2-3 minutes. Beat in the egg and vanilla.
Whisk together the flour, baking powder and salt in a separate bowl. Gradually mix the dry ingredients into the wet ingredients until just combined, being careful not to overmix.
Very gently fold in the fresh blueberries, trying not to break them up too much.
Portion out rounds of dough and place them spaced apart on a parchment-lined baking sheet. Use a spoon or piping bag to dollop some of the blueberry jam onto the tops of each dough round, then use a knife or toothpick to gently swirl it into the tops.
Remove the streusel from the fridge and generously top each dough round with streusel crumbs, gently pressing them in.
Bake at 375°F for 12-15 minutes, until lightly golden but still quite soft in the middles.
Allow to cool completely before enjoying! These are phenomenal served slightly warm with a glass of cold milk to dunk them into.
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