Optional: Soak your raisins in a bowl of hot water for at least 20 minutes.*
Combine warm water and milk in the bowl of a stand mixer fitted with a dough hook attachment**. Add the granulated sugar, then sprinkle the active dry yeast on top. Let sit for 5 minutes until the yeast is foamy.If the yeast does not foam, the yeast is dead, and you will need to start over with fresh yeast.
Add the flour, sourdough discard, and salt. With the mixer on low speed, slowly pour in the melted butter, then add the raisins. Mix until a rough dough forms. It should wrap around the dough hook and pull away from the sides of the bowl.
Transfer the dough to a smooth work surface. Knead for 4-5 minutes until a soft, smooth dough forms. If the dough seems sticky or stiff, add more flour or water (1 Tablespoon at a time) to reach your desired consistency.Place the dough in a large oiled bowl and cover with plastic wrap or a clean kitchen towel. Let rise for 1 ½-2 hours until doubled in size.
Grease a 9x5-inch loaf pan. Set aside.
Place the dough on a lightly floured work surface. Use a rolling pin to roll the dough into a 9x18-inch rectangle. It's important that this is only as wide as the width of your loaf pan (example: 9 inches wide for a 9x5-inch loaf pan).In a small bowl, combine the cinnamon and sugar. Brush the dough with milk, then sprinkle with the cinnamon sugar mixture, spreading evenly across the dough.
From the short side of the rectangle, roll the dough into a log. Think about this like rolling a burrito. Not too tight, not too loose, and the same thickness all the way across (not bigger on one side, etc.). Pinch the ends of the loaf together and along the seam to help keep the fillings on the inside of the loaf. Transfer the dough log into your prepared baking pan, placing the seam-side down, and tucking the ends of the loaf into the pan as best you can.
Loosely cover the pan with a greased piece of plastic wrap or a kitchen towel. Let rise 60-90 minutes, until the dough has risen about 1 inch above the edge of the pan. Note: the loaf will rise more as it bakes in the oven.
Preheat the oven to 350℉. Bake the loaf for 60-70 minutes until golden brown, and the internal temperature of the loaf reaches 190℉.
While the loaf is still warm, brush the top with 1 Tablespoon melted butter. Let the loaf cool in the pan for about 10 minutes, then transfer to a cooling rack to cool fully (about an hour) before slicing.
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Notes
1 serving = 1 slice*This is a great way to plump up raisins that are overly dried and might have been sitting in your pantry for a while! If you're working with fresh, plump raisins, you can likely skip this step. **You do not need a stand mixer to make this recipe. You can also mix the dough in a large bowl with a wooden spoon, then knead by hand. To make this recipe with active sourdough starter, omit the yeast and replace the sourdough discard with active starter (200g). You will also need to increase the rise times.