1. Heat oven to 400°F. Grease or line muffin cups (2-1/2-inches in diameter) with paper bake cups.
2. Stir together oats and buttermilk in small bowl; let stand 20 minutes.
3. Stir together oil, brown sugar, egg and vanilla in large bowl. Add oats mixture, stirring well. Stir together flour, cocoa, baking powder, salt and baking soda. Add to oats mixture, blending until moistened. Stir in nuts. Fill muffin cups 2/3 full with batter.
4. Bake 16 to 18 minutes or until wooden pick inserted in center comes out clean. Remove from pan to wire rack. Sprinkle muffin tops with powdered sugar, if desired. Serve warm or cool. Makes about 12 muffins.
* To sour milk: Use 1 tablespoon white vinegar plus milk to equal 1 cup.