This is the best lightened up Coffee Cake Recipe! You can use sour cream but I sub that with Greek yogurt and it is perfect. The smell is out of this world. Mix the dry ingredients with the wet until just combined (don't over stir) for the best result. This is tastiest warm from the oven, but leftovers are also delicious, it will hold up for about 5 days. 10 servings 3/4 cup old-fashioned rolled oats (about 2.5 ounces), divided Cooking spray 4.5 ounces all-purpose flour (about 1 cup) 1 ounce whole-wheat flour (about 1/4 cup) 1 teaspoon baking powder 1/2 teaspoon baking soda 1/4 teaspoon salt 1/2 cup granulated sugar 1/2 cup packed brown sugar, divided 1/3 cup butter, softened 2 large eggs 1 teaspoon vanilla extract 1 (8-ounce) carton light sour cream or Greek style Yogurt 2 tablespoons finely chopped walnuts, toasted 1/2 teaspoon ground cinnamon 1 tablespoon chilled butter, cut into small pieces 1. Preheat oven to 350°. 2. Spread oats in a single layer on a baking sheet. Bake at 350° for 6 minutes or until oats are barely fragrant and light brown. 3. Coat a 9-inch springform pan with cooking spray; set aside. 4. Reserve 1/4 cup oats; set aside. Place remaining oats in a food processor; process 4 seconds or until finely ground. Combine processed oats, flours, baking powder, baking soda, and salt; stir with a whisk. 5. Place granulated sugar, 1/4 cup brown sugar, and 1/3 cup butter in a large bowl. Beat with a mixer at medium speed for 3 minutes or until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Add flour mixture to sugar mixture alternately with sour cream, beginning and ending with flour mixture. (Batter will be slightly lumpy because of oats.) Spoon batter into prepared pan; spread evenly. 6. Combine remaining 1/4 cup oats, remaining 1/4 cup brown sugar, nuts, and cinnamon in a bowl. Cut in 1 tablespoon butter with a pastry blender or 2 knives until well blended. Sprinkle top of batter evenly with nut mixture. Bake at 350° for 38 minutes or until a wooden pick inserted in center comes out clean, top is golden, and cake begins to pull away from sides of pan. Cool cake in pan for 10 minutes; remove from pan.