Sourdough Coffee Cake with Walnut Streusel

• In the bowl of a stand mixer with the paddle attachment or a large mixing bowl with a wooden spoon, combine 8 oz active sourdough starter, 4 oz whole milk and 1 cup of the flour. Mix until it forms a thick batter. Cover the bowl and set aside for 30-60 minutes.
• Add 3 oz granulated sugar, 2 oz unsalted butter (melted & cooled), 1 large egg, 1 teaspoon vanilla extract, ½ teaspoon salt and ½ teaspoon cinnamon to the bowl. Mix on low speed to combine. If working on a stand mixer, switch to the dough hook. With the mixer running, add the remaining flour until the dough gathers on the hook and clears the sides of the bowl. If working by hand mix as much flour as you can with the spoon, then finish kneading in the rest of the flour by hand. If the dough is very sticky sprinkle in a little more flour as you knead.
• Turn the dough out onto a floured surface and knead to a smooth ball. The dough should be soft and silky.
• Put the dough into a lightly oiled bowl, turning once to coat the dough. Cover the bowl and set it aside at room temperature.
• After 60 minutes uncover the bowl, lift one side of the dough and fold it into the middle of the dough. Repeat with the other three sides of the dough then flip the dough over. You're basically turning the dough inside-out to redistribute the yeast. Cover the bowl and after 60 minutes repeat the procedure again. Cover the bowl and after 60 minutes the dough should be ready to shape. The dough should be lively and aerated. If it still seems sluggish give it some more time to ferment. The total time will vary based on dough temp, air temp and how active your starter was.
• While the dough ferments, prepare the streusel.

Prepare the Streusel/Crumb Topping

• Combine 3 oz brown sugar, 3 oz granulated sugar, 4 oz walnuts, 2 teaspoons ground cinnamon and ⅛ teaspoon table salt in a food processor. Process until the nuts are chopped coarsely but not ground to a powder. Remove 2/3 cup for filling and set it aside.
• Add 2.5 oz all purpose flour and 2 oz unsalted butter to the remaining streusel and process until it forms clumps. Set it aside to use later as the crumb topping.

Assembly

• Generously butter a 10" tube pan. Roll the dough to a 12"x 16"rectangle. Brush the surface of the dough with egg wash. Sprinkle the streusel over the dough. Roll the dough into a log, pinch the seam to seal.
• Bring the two ends of the log together to form a ring. Pinch the ends together.
• Transfer the dough to the prepared baking pan. Cover the pan and place the cake in the refrigerator overnight.
• The next morning, take the cake out of the refrigerator. The cake should rise in the refrigerator overnight, but it is probably not fully proofed. It should be almost doubled in volume. Leave the cake at room temperature to finish rising, it may take 1-1.5 hours.
• Meanwhile, preheat the oven to 350 °F. To speed up the rising you can put the pan in the oven, VERY BRIEFLY, while it's just starting to warm up.
• Brush the surface of the cake with egg wash. Grab large chunks of the crumb topping and break them into smaller chunks over the surface of the cake.
• Bake until the cake is well risen and golden brown, about 30 minutes. You can use a probe thermometer to check the internal temperature. It should be about 200 °F.
• Cool the cake for 20 minutes in the pan, then turn out onto a cooling rack to cool completely.
• Combine 4 oz confectioner’s sugar with 1 oz whole milk and 1 teaspoon vanilla extract to form an icing with the texture of thin pancake batter. Drizzle the glaze over the cake and allow it to set.