Place a rack in the centre of your oven and preheat the oven to 350°F.
Lightly grease a 9x13 inch baking pan and line it with parchment paper making sure to leave a few inches of paper hanging over opposite sides of the pan. Doing this will make it easier to lift the finished bars out of the pan.
In a medium bowl, add the flour (starting with 3 cups (add up to 1/2 cup more if the dough is too sticky), baking powder and salt, whisk and set aside.
Melt the butter in the microwave.
In a large bowl, add the melted butter and granulated sugar. Whisk vigorously until butter and sugar are well combined.
Add the eggs, one at a time, whisking after each addition until the egg is blended into the batter.
Add vanilla and lemon zest and whisk into batter.
Add one-third of the flour mixture and whisk to incorporate into batter, repeat with each third until all flour is mixed in.
If the dough remains too sticky, go ahead and add that extra half cup of flour. Mix it in the dough and divide the dough into two portions, one just a bit larger than the other.
Keep the larger half to work with immediately and place the smaller portion in the refrigerator until ready to use.
Press dough onto the bottom of the pan. Gently press down on surface to make sure that the bottom is even.
Spread blueberry jam on bottom half of the dough and sprinkle with the chopped hazelnuts.
Remove the remaining dough from the refrigerator, working with a bit at a time and using the large holes of a box grater, run the dough across the grater to create a crumbled topping.
Spread the topping evenly across the top of the bars.
Bake until the edges of the bars are golden, about 30 minutes. If surface of bars isn't golden place it under the broiler for about two minutes until the top is golden.
Cool on a rack for 20 minutes and then pulling up on sides of parchment paper lift bars out of the pan onto the cutting board and cut into evenly sized bars.
The bars can be stored in the refrigerator for 5 days, or frozen for later consumption.