Preheat oven to 350 degrees F. Grease twelve 2 1/2-inch muffin cups; set aside.
Place butter in a small heavy saucepan. Cook over medium heat about 15 minutes or until butter is melted and lightly browned. Pour the browned butter into a small heat-proof bowl and set aside to cool slightly.
Meanwhile, in a large bowl whisk together the next 6 ingredients (through baking soda). In a medium bowl whisk together the cooled browned butter, egg, and buttermilk. Make a well in the center of the flour mixture. Add buttermilk mixture to flour mixture and stir to combine. Spoon batter into muffin cups, filling each 3/4 full. Brush both sides of each fresh sage leaf lightly with oil and place one or two on top of the muffin batter in each cup.
Bake about 20 minutes or until lightly browned. Cool in muffin cups 5 minutes. Remove and cool slightly on wire racks. Serve warm.