Spread the hazelnuts over a baking pan and bake in the oven at 177°C (350°F) for about 25 minutes. Hazelnut skins will get very dark and cracked. Wrap the warm nuts in a kitchen towel and cool for 20 minutes. Rub the towel with nuts in a circling motion against the palm of your hand or a table top. This will remove almost all the skins and don’t worry if some skin remains. This will add to the colour and flavor.
Crust
Position the rack in the lower third of the oven and preheat to 177°C (350°F).
Line 20 or 23 cm (8″ or 9″) square baking pan with parchment paper.
In a food processor pulse together flour, sugar, salt and toasted and skinned hazelnuts until hazelnuts are ground.
Melt the butter in a medium size sauce pan over medium heat.
Add melted butter to the processor and pulse until dry ingredients are damp and mixture begins to clump around the blade. Remove the dough from processor and kneed briefly until smooth.
Pressing with the palms of your hands spread the dough evenly across the pan.
Bake for 30 minutes or until the crust is golden brown.
Topping
In a medium size bowl mix together sugar and flour; whisk in the eggs.
Mix in apricot preserves making sure you break down any lumps.
Stir in the lemon juice.
Assembly
When the crust is baked reduce the temperature to 150ºC (300ºF). Without removing pan from the oven pour the filling over the hot crust.
Bake for 20-25 minutes or until topping doesn’t jiggle when the pan is tapped on the side.
Set the pan on a cooling rack and let cool completely.
Lift the parchment paper liner and transfer the cake to the cutting board. Cut the cake into 16 square bars. Sift confectioner’s sugar over the squares before they are served.
Store the squares refrigerated in airtight container.