Preheat the oven to 375°F. Grease a muffin tin with butter or spray oil.
Cut the butter into chunks and melt gently. Keep the butter liquid, but cool(ish) as you do the rest.
Mix the flour, baking soda, baking powder and salt in a large bowl. In another bowl, mix the honey, sugar, egg, sour cream and melted butter. Whisk this constantly as you add the butter so it doesn't cook the eggs.
Pour the wet into the dry, stirring all the while. Add the rhubarb and nuts and keep stirring and mixing them all in. When you have a thick batter, spoon it evenly into the muffin tin and bake for 20 to 25 minutes, or until a toothpick stuck into the center of a muffin comes out clean.
Set the tin on a cooling rack for 5 minutes before you pop them out of the tin to cool further.