Mix the dry ingredients with the wet until just combined (don't overstir) for the best result. This is tastiest warm from the oven, but leftovers are also delicious.
10 servings (serving size: 1 piece)
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. Weigh or lightly spoon flours into dry measuring cups; level with a knife. 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.