make the whole wheat cookies dough
Add all-purpose flour, whole wheat flour, ground hazelnuts, raw sugar, salt, baking powder, and lemon zest to a large bowl (or use a food processor). Add cold butter cubes. Using your fingertips, rub the butter into the flour mixture until there are no lumps or chunks of butter left. Add molasses and milk to the flour mixture and knead until it all comes together, for about 30 seconds.
place the dough in the fridge
Divide the dough in half and shape it into a rectangle. Wrap in cling film and place in the fridge for 30-45 minutes (or overnight). Preheat the oven to 180 °C / 350 °F.
cut the cookies and bake
Remove one half of the cookie dough from the fridge. Place a sheet of baking paper on the working surface and place the cookie dough on top. Lightly dust the dough with flour, then cover with another sheet of baking paper. Using a rolling pin, roll the dough into a rectangle about 1-2 mm or 1/20 inch thick. Using a Linzer cookie cutter (5 cm / 2-inch), cut cookies from the dough. Transfer them to a baking sheet, lined with baking paper. Bake for about 12 - 14 minutes at 180 °C / 350 °F, or until golden brown. Transfer to a rack to cool completely. Repeat the process with the remaining dough.
fill and serve
Turn the cookies without a hole in the middle, flat side up. Spoon one teaspoon of Nutella into the center of a cookie, spreading it slightly. Then cover with the cookie that has a hole in the center to create a delicious cookie sandwich. Place the sandwich cookies on a nice serving platter and serve. Store them in an airtight container for up to 14 days.