Tip 275g of the flour into a mixing bowl. Add the yeast to one side of the bowl and the 25g of sugar and the salt to the other side. Crack the eggs into a separate small bowl, add the milk and beat together with a fork.
Make a well in the centre of the flour and pour in the egg and milk mixture.
Using a table knife, mix the wet ingredients into the dry ingredients to form a flaky dough.
Tip out the dough onto a work surface and bring it together with your hands. Knead it for 5-8 minutes, until smooth.
Pat the dough into a rough 15cm round and add 4 or 5 cubes of butter to the centre. Fold the dough over the butter and knead to incorporate. Repeat, kneading well between each addition, until you have used all 80g of the butter and have a soft, slightly sticky dough.
Once you have added all the butter, continue to knead through the sticky stage, until you have a ball of smooth, silky, shiny dough.
Lightly oil a clean mixing bowl and place the dough inside, cover with a clean tea towel and leave at room temperature to rise for 1 hour, until doubled in size.
Tip out the risen dough onto a work surface, pat it down and press out the air all over with your fingertips.
Sprinkle some of the remaining 25g of flour over your work surface (you may not need it all), place the dough on top and, using a rolling pin, roll it out to about a 30 x 35cm rectangle.
Using the back of a spoon, spread the chocolate mixture over the dough, leaving a 1cm border around the edges. Sprinkle both the roughly and the finely chopped hazelnuts evenly over the top.
Using your hands, roll up the dough tightly, keeping the seam underneath as you finish. Turn the roll through 90° to position it vertically on the work surface, like a clock hand pointing to the 12 o'clock.
Trim about 2cm off each end and discard the trimmings. Using the large knife and starting at the end of the roll farthest from you, cut through the middle, down the full length, dividing the roll into two long pieces.
Turn each piece so that the cut side faces upwards. Gently press the top ends of the two halves together to seal. Then, lift the right half over the left half followed by the left half over the right half, repeating the process to make a two-stranded plait. When you get to the bottom, gently press the bottom ends together to seal.
Carefully lift the plait into the prepared loaf tin, squishing the length to fit if necessary, and cover with a tea towel. Leave to rise again at room temperature for 1-2 hours, until puffed up and doubled in size.
Heat the oven to 190°C/170°C fan/Gas 5. Once the babka has risen, bake it for 15 minutes, then reduce the oven heat to 170°C/150°C fan/Gas 3 and bake for a further 25-30 minutes, until a skewer inserted into the centre comes out clean. Leave in the tin on a wire rack while you make the syrup.