Preheat the oven to 375. Line a baking pan with parchment and spray a muffin tin with non-stick spray.
Whisk together the brown sugar, melted butter and water, dividing evenly in the prepared muffin tin(s). Layer with the sliced nectarines.
In a large bowl, mix together the bran, flour, brown sugar, Davis Baking Powder, baking soda and salt. Make a well in the center and add the buttermilk, oil, beaten eggs and vanilla. Whisk thoroughly yet briefly, to avoid making the batter tough.Spoon over the nectarines in prepared muffin tins.
Bake for about 20 to 25 minutes, depending on size of the muffin tins. Cool about 5 minutes, run a knife around the edge of each muffin and invert. If any of the fruit slips off, nab it with a fork and place it back on the inverted muffin; it will adhere while cooling.