Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, cream together unsalted butter and granulated sugar until light and fluffy using a mixer fitted with the paddle attachment.
Add the egg, lemon juice, and lemon zest; mix until combined.
In a separate bowl, whisk together purpose flour, baking powder, and salt. Gradually mix the dry ingredients into the wet mixture until just combined.
Gently fold in the fresh blueberries and white chocolate chips (if using).
Scoop out dough using a spoon and form into small dough balls. Place them on the prepared baking sheet.
Bake for 10-12 minutes or until the edges are golden brown. Let cookies cool on a wire rack.
For the lemon glaze, whisk together powdered sugar and lemon juice until smooth. Drizzle over cooled cookies.