Prepare muffin tins by spraying with nonstick spray, or lining the tins with cupcake liners (I like to spray my liners with nonstick spray as well).
Mix together brown sugar, flour, and pecans. Add melted butter and eggs to mixture.
Pour batter into prepared muffin tins.
Place a pecan half on top of each muffin, if desired, and lightly press it into the batter.
Bake at 350° F for 15 - 20 minutes for twelve regular size, or 10 - 12 minutes for miniature muffins. Remove from oven, and let muffins cool thoroughly.