To make the brioche dough, put the flour, milk, yeast, and sugar into the bowl of an electric mixer fitted with a dough hook. Mix on medium speed until well combined, then add the eggs one by one and mix again. When the dough starts to come away from the sides of the bowl, start adding the butter, dice by dice, ensuring that each addition is well incorporated. Cover and leave to rise in a warm place (over 70℉/21℃) for 1½-2 hours. It will be too soft to work with at this stage--refrigerate for 2-3 hours or overnight and it will be ready to roll.
To make the filling, thoroughly combine all the ingredients together to form a smooth mixture.
Divide the dough in two. Line a baking sheet with parchment paper and grease a 10 inch (25 cm) cake ring or springform pan with its base removed. Using a a rolling pin, roll half of the dough into a circle slightly larger than the ring or pan, and place it onto the baking sheet. Place the ring or pan on top of the pastry circle (you will trim the excess later).
Divide the remaining half of the dough in two. Roll into two ropes, measuring about 34 inches (87 cm) long. Loosely twist the dough ropes together, leaving room for the dough to expand. Place inside the ring or pan, creating a beautiful twisted border around the sides.
Preheat the oven to 375℉ (190℃). Brush the twisted border with the egg wash, then pour the filling inside the border. Scatter the orange zest on top and dot with the diced butter.
Bake for 25-30 minutes until the filling is soft and wobbly in the middle. If the dough becomes too dark during baking, cover with foil. Cool on a wire rack before trimming any excess dough outside the cake ring. Remove the cake ring by running a knife along its edge to loosen it, then lift away. If using a springform pan, simply open it to release from the cake.
Serve for dessert or as a treat for breakfast. If you have any raisins left, scatter them on top.