Heat the milk in a small pan over a low heat until warm, but not hot. Set aside. Put the flour into the bowl of a stand mixer. Add ½ tsp salt to one side of the bowl and the sugar and yeast to the other. Mix each side into the flour with your hands, then using the dough hook attachment until fully combined.
Pour in the warm milk, then, with the mixer on medium, add the eggs one at a time. Keep mixing the dough for 10 mins until smooth, then gradually add the butter, one or two cubes at a time, until fully incorporated, about 5-8 mins. Scrape down the sides of the bowl – the dough will be very soft.
Scrape the dough into a large bowl, cover with a clean tea towel and leave to rise for 1 hr 30 mins-2 hrs until doubled in size. Once doubled, chill for 1 hr.
Line a large baking sheet with baking parchment and set aside. To make the filling, put all the filling ingredients and a large pinch of sea salt flakes in a small saucepan over a low-medium heat, stirring continuously until everything has melted together. Transfer to a bowl, leave to cool completely, then transfer to the fridge and chill for 40 mins, stirring every 20 mins until the mixture has thickened but is still spreadable.
Remove the dough from the fridge, and roll out on a lightly floured surface to a roughly 70 x 40cm rectangle. Spread the filling evenly over the dough using a spatula or palette knife. With a shorter end closest to you, roll the dough up into a tight sausage. Transfer to a board, seam-side down, and cut in half across the length so you have two long pieces. Lay the two pieces out in front of you, parallel to one another, and cross them over each other along the length in a plait-like pattern. Curl into a circle and join the ends to make a wreath, then carefully transfer to the lined sheet.
Cover with a tea towel and leave to rise for 1 hr until doubled in size. Heat the oven to 180C/160C fan/gas 4 and bake the babka for 35-40 mins until golden.
For the syrup, simmer the sugar, orange juice and zest over a high heat for 5-10 mins until thickened. Brush the babka with the warm syrup. Strain the candied orange zest and scatter over the babka. Serve warm, or leave to cool completely, then keep wrapped. Will keep wrapped at room temperature for up to three days.