Preheat oven to 350F/180C. Start by browning the butter. Add the butter to a small saucepan and start heating it over medium heat. The butter will go through several stages before it will turn into “brown butter”, be patient and don’t walk away from it. First your block of butter will melt, then it will turn frothy. Start stirring it once you see the foam, which will eventually go down and reveal brown, nutty smelling melted butter. Once you see that, take it off the heat and cool. Once your brown butter is at room temperature or close enough, mix in brown sugar, eggs and vanilla.
In a small bowl mix the flour, baking powder and salt, whisk together.
Mix flour mixture into sugar mixture until combined. Mix in 1 cup coarsely chopped hazelnuts and pour into a greased 13 x 9 inch baking pan. Spread batter evenly.
Bake for about 30 minutes, or until a toothpick inserted comes out clean, careful not to over bake, blondies will firm as they cool. Cool pan on top of a wire rack.
Melt the white chocolate in a double boiler or in a microwave in a few 30 second spurts as it burns very quickly. Check after each time, stir and put it back in the microwave for 30 seconds longer until the chocolate is runny.
Use 1/3 of the melted chocolate for drizzling over the blondies. You can do it with a spoon or transfer the chocolate into a small plastic bag (sandwich bag), then cut a tiny corner off and drizzle through the opening.
Dunk 1/2 cup whole hazelnuts in the remaining chocolate, make sure they are fully coated, then take them out with a fork and place on a piece of parchment paper and allow the chocolate to set until firm. Scatter the white chocolate covered hazelnuts all over your blondies before serving.