Place the water, butter, and milk in a heat-proof bowl or jug and heat until the butter is melted. Allow the mixture to cool until it is lukewarm.
250 grams water, 4 teaspoon instant yeast, 125 grams milk, 90 grams butter
Add the egg, sugar, and salt to the butter mixture and whisk to combine.
1 egg, 80 grams white granulated sugar, 1 teaspoon table salt
Combine 600g of flour and the yeast in a large mixing bowl. Add the liquids to the dry ingredients and mix using a fork or spoon until most of the flour is mixed into the liquids. Let this mixture then rest for 15 minutes, allowing the flour to become hydrated.
750 grams bread flour
Using your additional cup of flour, flour the surface you are going to knead your dough on. Place the dough on the counter and toss the rest of the flour over your dough. Knead for about 10 minutes until the dough is soft, pliable, and forms a smooth ball. *Note 1
Grease or oil a large mixing bowl. Put the soft dough into the bowl and cover with a tea towel, in a warm place to rise for about an hour.
While the dough is rising, make the filling by combining the brown sugar, spices, and butter in a heat-proof bowl and microwave until the butter has melted. Whisk until the sugar is dissolved and the mixture is smooth and shiny. Set aside and allow to cool.
100 grams light brown sugar, 2 teaspoon ground cinnamon, ¼ teaspoon ground nutmeg, 60 grams butter
When the dough has almost doubled in size, roll it out on a lightly floured work surface in a rectangle about 40cm long and 25 cm wide. Spread the filling over the dough using a spatula or the back of a spoon. Sprinkle the filling with the raisins and mixed peel.
¼ cup mixed peel, ¾ cup raisins
Beginning on the long edge of the dough, roll the dough up tightly. Pinch the seams together to seal in the filling along the length of the dough. Cut the roll into 12 equal portions using a sharp knife, or bench scraper.
Place the rolls cut side down into your baking dish (9x13inch) leaving about 2 - 3 cm of space between the rolls to allow them to rise.
Cover the rolls with a tea towel and allow to rise for about an hour. Preheat the oven to 180°C/355F. Then bake the rolls for 25-30 minutes.
While the buns are baking, let the butter and sour cream come to room temperature, you may need to pop them into the microwave for 10 seconds if they are too cold for creaming.
Add the icing sugar and lemon juice to the butter and sour cream and whip until well combined using either a stand mixer, electric mixer, or whisk. You can frost the buns as soon as they come out of the oven, or you can allow them to cool a bit first.
300 grams icing sugar, 40 grams butter, 60 grams sour cream, 2 teaspoon lemon juice
Chelsea buns can be served warm or at room temperature. Enjoy!