Sourdough Coffee Cake with Walnut Streusel

Name: Sourdough Coffee Cake with Walnut Streusel
Prep time: 45 min
Cook time: 30 min
Total time: 1 h 15 min
Cuisine: European, German
Ingredients: 1 cup (8 oz, 224g) active sourdough starter (100% hydration); 1/2 cup (4 oz, 120 ml) whole milk, warmed to 100°F; 2 cups (10 oz, 280g) all-purpose flour; 1/3 cup (2.5 oz, 70g) granulated sugar; 1/2 stick (2 oz, 56g) unsalted butter, melted and cooled to room temperature; 1 large egg, room temperature; 1 teaspoon vanilla extract; 1/2 teaspoon salt; 1/2 teaspoon cinnamon; 1/3 cup (2.5 oz, 70 grams) brown sugar; 1/3 cup (2.5 oz, 70 grams) granulated sugar; 1 cup (4 oz, 115 grams) walnuts; 2 teaspoons ground cinnamon; pinch of salt; 1/2 cup (2.5 oz, 70g) all purpose flour; 1/2 stick (2 oz, 55g) unsalted butter, softened to room temperature; Butter for pan prep; 1 egg, whisked; 1 cup (4 oz, 112g) confectioner's sugar; 2 tablespoons (1 oz, 30ml) whole milk