Step 1: Prepare the Peach Filling: In a medium bowl, combine sliced peaches, 1/4 cup granulated sugar, cornstarch, lemon juice, and 1/4 teaspoon ground cinnamon. Toss gently to coat. Set aside.
Step 2: Prepare the Streusel Topping: In a small bowl, whisk together 1/2 cup all-purpose flour, brown sugar, 2 tablespoons granulated sugar, and 1/2 teaspoon ground cinnamon. Cut in the cold unsalted butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Refrigerate until needed.
Step 3: Preheat Oven & Prepare Pan: Preheat oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan or line with parchment paper, leaving an overhang on the long sides.
Step 4: Combine Dry Ingredients: In a large bowl, whisk together 1 3/4 cups all-purpose flour, baking powder, baking soda, 1/2 teaspoon ground cinnamon, ground nutmeg, and salt.
Step 5: Cream Wet Ingredients: In a separate medium bowl, cream together the softened unsalted butter and 1 cup granulated sugar with an electric mixer until light and fluffy, about 2-3 minutes.
Step 6: Add Eggs & Vanilla: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 7: Combine Wet and Dry: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined. Do not overmix.
Step 8: Assemble the Loaf Cake: Pour about half of the cake batter into the prepared loaf pan and spread evenly. Arrange about half of the peach mixture (try to avoid excess liquid) over the batter. Gently spoon the remaining batter over the peaches, spreading it as evenly as possible.
Step 9: Top with Peaches & Streusel: Top the remaining batter with the rest of the peach mixture, distributing it evenly. Sprinkle the chilled streusel topping generously over the peaches.
Step 10: Bake: Bake for 55-70 minutes, or until a wooden skewer inserted into the center of the cake comes out with moist crumbs, but no wet batter. If the top browns too quickly, loosely tent with aluminum foil.
Step 11: Cool: Let the cake cool in the pan on a wire rack for 15-20 minutes before carefully lifting it out by the parchment paper overhangs. Transfer to the wire rack to cool completely before slicing and serving.