For best results, use ripe but firm pears. Bosc or Anjou varieties work well. Buttermilk adds a lovely tang and helps to create a tender crumb. If you don't have buttermilk, you can make a substitute by adding 1 tablespoon of lemon juice or white vinegar to a cup of milk and letting it sit for 5 minutes. Don't overmix the batter! Overmixing develops the gluten in the flour, resulting in tough muffins. These muffins are best enjoyed warm. Store leftovers in an airtight container at room temperature for up to 2 days.