Place the milk in a small saucepan and heat up until it is lukewarm. Add cubed butter and take of the heat. Swirl the saucepan few times, to encourage the butter to melt. Set aside and allow all the butter to melt.
In a large bowl of a freestanding electric mixer, mix together the flour, sugar, yeast and salt. Make a well in the middle, and add lukewarm milk and melted butter and large egg. With mixer running on low speed, combine these ingredients together for 2 minutes. Increase the speed to medium- high and mix for further 6-8 minutes until you have smooth and elastic dough, that no longer sticks to the sides of the bowl. Remove the dough from the bowl, and using your hands shape it into a ball.
Lightly grease a large bowl with some olive oil, then place the dough in the bowl and cover with clean kitchen towel, cling film or shower cup. Set aside (ideally, somewhere warm), and let it rise for 2 hours or until double in size.
To make the filling, place the softened, cubed butter, brown sugar and cinnamon in a bowl of a standing mixer fitted with paddle attachment and beat until well combined and fluffy, about 3 minutes. Set aside. Line the 10' round baking tin or dish with baking paper and set aside. Line the large baking tray with baking paper and set aside too.
Remove the dough from the bowl into a lightly floured surface. Using your fingertips, gently knock the air out of the dough and stretch it into a small, rectangular shape. Using the rolling pin, roll the dough into a large rectangle, about 35 x 50 cm. With the longer edge of the rectangle facing you, spread the cinnamon butter over the whole surface of the dough. Top with chopped, toasted hazelnuts.
Starting from the edge closest to you, roll the dough into a tight log. Transfer the log into a large baking tray lined up with baking paper and place it in the fridge for 1 hour.
Remove the dough from the fridge and place it onto a lightly floured surface. Using a sharp knife, cut the dough log in half, lengthwise, leaving a small section in tact/ still attached at one end. Turn the halves carefully, filling side up. Cross the dough halves over each other, repeating until you have a long, twisted piece of dough.
Cross one end of the twist over the other, tucking the ends underneath and forming a knot. Transfer the knot into the lined up round baking tin/ dish, cover with clean kitchen towel and allow it to rise for 45 minutes- 1 hour.
Pre heat the oven to 180 C (170C fan). Brush the dough with egg wash and bake for 45- 55 minutes, until golden brown. If the top starts to brown up too much, cover it with some kitchen foil for the last 10 minutes of the bake. Remove from the oven and allow to cool slightly. Serve while still warm, enjoy!