For topping, in a small bowl, cut butter into granulated sugar until mixture resembles coarse crumbs; set aside.
For muffins, line muffin cups with paper bake cups; set aside. In a large mixing bowl, combine egg, brown sugar, oil, honey and vanilla.
In another bowl, stir together the flour, baking soda, and salt. Add dry ingredients to the egg mixture alternately with sour milk, stirring after each addition until moistened (batter should be lumpy). Gently fold in the chopped rhubarb and the chopped nuts.
Fill muffin cups 2/3 full with batter. Sprinkle with topping. Bake in a 325 degree F oven for 35 to 40 minutes or until golden. Remove from pans. Serve warm or cool with Whipped Honey Butter. Makes about 21 muffins.
To make sour milk, place 1 tablespoon vinegar in glass measure. Add enough milk to make 3/4 cup.
In a small mixing bowl, combine butter and lemon peel. Gradually add honey, beating with an electric mixer on high speed until fluffy. Store, covered, in the refrigerator. Let butter spread stand at room temperature for about 1 hour before serving.