In a medium bowl, stir together the yogurt, 2 tablespoons of melted butter, oil and egg. In a large bowl, stir together the flour, 3/4 cup of brown sugar, baking soda and salt. Pour the wet ingredients into the dry, and mix until just blended. Fold in rhubarb. Spoon into the prepared muffin tin, filling cups at least 2/3 full.