Place sweet potato, brown sugar, orange juice, rosemary and 2 tablespoons butter in food processor. Puree until smooth. Add milk and egg, and mix to combine. In a medium bowl, combine flour, baking powder and salt. Add sweet potato mixture and stir until combined. In a large nonstick skillet, melt 2 tablespoons butter over medium heat. Add sweet potato batter – 1 tablespoon for each blini – and make into small round shape. Cook until golden on each side. Let cool. When ready to serve, place blinis on platter and top with a dollop of crème fraiche and caviar.