Preheat oven to 375°F. Spray 8 large muffin cups (1-cup capacity) or 18 standard muffin cups (⅓-cup capacity) with nonstick spray, or line them with paper muffin liners. (Note: I baked off these muffins in paper-lined ramekins. It worked beautifully.)
Whisk oats and next 9 ingredients in large bowl. Add buttermilk, oil, egg, and vanilla; whisk to blend. Stir in ⅓ cup boiling water and let stand 5 minutes. Divide batter among prepared muffin cups.
Bake muffins until tester inserted into center comes out clean, about 28 minutes for large muffins and 20 minutes for standard muffins. Cool 10 minutes. Turn muffins out onto rack; cool. Serve warm or at room temperature.
