• Note: this dough does best when given ample time to rest in the fridge. I like to make mine the night before, leave it for an hour at room temperature, then in the fridge overnight, to shape in the morning.
• Make the tangzhong: set the milk, water and flour in a small pot. Cook over medium heat, stirring frequently, until it comes together in a loose paste, this will take about 3-5 minutes. Take off the heat and set aside.

To make the doughs

• In the bowl of a stand mixer, add the water and instant yeast. Let sit for a few minutes, it should foam (this indicates the yeast is active). Add the flour, sugar, eggs, milk or buttermilk powder, salt and the tangzhong from above. Affix the dough hook and begin kneading on low, working up to a low medium speed. When the dough starts to come together around the dough hook, add the butter in small pats (about a teaspoon at a time). Knead on medium speed until the dough gathers around the hook again, though it will still be quite loose and tacky.
• Remove half of the dough into an oiled bowl (this will be the cocoa dough). To the half in the mixing bowl, add another 2 tablespoons of all purpose/bread flour (this is the plain dough). Knead until it comes together around the dough hook. It should be a lot less sticky now. Transfer it to an oiled bowl and cover for a first rise.
• Return the first half of the dough that was set aside and add the cocoa and start to knead. As it’s kneading add a tablespoon of water, if it still looks like it needs it, add another tablespoon but no more than that. Once it is well mixed, transfer it back to its oiled bowl, cover and leave to rise.
• The first rise can be on the counter at room temperature until both doughs are almost doubled (but not fully doubled). Alternatively, you can make the doughs the night before, leave them at room temperature for about an hour then set them in the fridge overnight. Whichever option you choose, it’s best to have the dough for at least 30 minutes in the fridge before trying to shape the loaf to make it easier to handle.

To shape into the swirl/marble loaf

• On a very well floured countertop, split both doughs in half. Roll each half (for a total of four) into a long rectangle, about 16 inches long and 9 inches wide. Layer the rolled out doughs on top of each other: I start with plain, then chocolate, then plain, and lastly chocolate. Roll the dough up from the shorter side so you have a short log. Set the dough in an oiled pullman loaf pan (or a large loaf pan, I use a 10x5 inch pan). Cover for a second rise. When the dough is quite puffy and has risen about an inch above the rim of the pan, try poking it gently: if it springs back it needs more time to rise. If you leave an indentation that rises back slowly (or barely) the loaf is ready to bake.
• Preheat the oven to 375 F. Set the pan in the oven on the middle rack and bake for 10 minutes. After that 10 minutes, lower the temperature to 350 F and bake for another 30 minutes or so: the bread is done when a thermometer inserted into the center registers 190 F. Let cool briefly then transfer to a cooling rack.
• Store the bread in an airtight container, it’ll be good for a few days. Freeze leftovers!