Preheat your oven to 160 degrees Celsius, (320 Fahrenheit)
Defrost your blueberries, I do this in the microwave at 50% power for 2 mins.
Cream the butter and sugar, keep beating until the creamed butter and sugar is pale. I use my stand beater for this.
Blitz the blueberries, you can either do this by adding it to your creamed butter and sugar and let your beater pulverise the blueberries. I find I get a better result if I blend them in my smoothie maker and then add them to the cake mixer.
Beat in the lemon juice, the more lemon juice the more vibrant the colour of the bars will be, but obviously the more you use the stronger the lemon flavour will be in the blueberry bars.
Whisk or sift the flour, salt and baking powder together so that they are well combined. Make sure you do this, as if the baking powder isn't well distributed through the flour you can end up with grey blueberry cookies.
Add the dry ingredients to the creamed blueberries and mix until combined.
Stir in two thirds of the the white chocolate chips
Press the blueberry cookie dough into the bottom of a lined cake tin using a spatula. Dampen your spatula as this will stop the cookie dough from sticking. You want the cookie dough to be around 1.5cm-2cm thick.
Sprinkle the remaining chocolate chip on top of the blueberry cookie dough
Bake for 20 minutes until the blueberry cookie dough is lightly golden at the edges of the cake tin. Cool in the tin, then slice into bars or square.