Preheat oven to 160°C fan / 350°F / Gas 4 and line a 20cm (8 inch) square baking pan with baking paper.
In a small saucepan, melt the butter.
In a medium bowl add the brown sugar and whisk in the melted butter. Mix well for about a minute to aerate the mixture.
Add the vanilla and eggs and mix well.
In to large bowl, sift the flour, salt and baking powder.
Chop the white chocolate into small chunks and add 2/3 of it to the flour mix with the blueberries and gently stir together.With a spatula, fold in the dry ingredients to the wet ingredients and stir until just combined. Do not overmix. Pour the batter into your prepared baking tray and smooth the top with a rubber spatula.
Scatter the remaining chocolate pieces over the top.
Baking: Place on the middle shelf of the oven for 30-35 minutes until a toothpick inserted in the centre of the blondies comes out clean. Place the pan on a wire rack and allow the blondies to cool completely in the baking tray/pan.
Using a large knife, cut the blondies into 9, 12 or 16 equal squares. If you want the blondies to be extra neat, you can trim all four sides.