To make the dough: Place flour, sugar, and yeast in the bowl of a standing mixer fitted with the dough hook and mix on low speed until combined. Add eggs, water, and lemon zest, and mix until dough comes together, 2-3 minutes. While mixer is running, add butter, a few cubes at a time, and mix until incorporated, about 5 minutes. Add salt and continue mixing for another 5 minutes on low speed, until the dough is completely smooth and elastic and pulls away from the sides of the bowl. During mixing, scrape down the sides of the bowl using a spatula as needed.
Grease two 2¼-lb/1kg loaf pans (9×4 inch/23×10 cm) with oil and line the bottom of each pan with wax paper for easy release later on. Set aside.
Rolling and shaping (photos here): Divide dough in half. Roll out dough on a lightly floured surface and shape into a rectangle measuring 16×14 inches (40×35 cm). Position dough so that the long side is closest to you. Using an offset spatula, spread half of the Nutella spread over the rectangle, leaving a ½-inch/1cm border all around. Sprinkle half of the chocolate chips or nuts on top.