• Center a rack in the oven and preheat to 375°F.
• Line a 12-cup muffin tin with paper liners or spray with cooking spray. Place the muffin pan on a baking sheet.

Make the Streusel:

• Put the flour, brown sugar, and allspice in a small bowl and sift ingredients through your fingers to blend. Add the cold pieces of butter and toss to get irregularly shaped crumbs. Set aside in refrigerator.
  ½ cup all purpose flour, ½ cup light brown sugar, ½ teaspoon ground allspice, 5 tablespoons cold unsalted butter

Make the Muffins:

• In a large bowl, whisk together the flour, sugar, baking powder, allspice, and salt. Stir in the brown sugar, making certain there are no lumps.
  2 cups all purpose flour, ½ cup sugar, 1 tablespoon baking powder, ½ teaspoon ground allspice, ¼ teaspoon salt, ¼ cup light brown sugar
• In a large glass measuring cup or another bowl, whisk the melted butter, eggs, milk, and vanilla extract together until well combined.
  8 tablespoons unsalted butter, 2 large eggs, ¾ cup whole milk, ¼ teaspoon vanilla extract
• Pour the liquid ingredients over the dry ingredients and, with a whisk or rubber spatula, gently but quickly stir to blend. The batter should be lumpy. Stir in lemon zest if using.
  zest of 1 lemon
• Divide the batter evenly among the muffin cups. Sprinkle some streusel over each muffin, then use your fingertips to gently press the crumbs into the batter.
• Bake for about 20 minutes or until the tops are golden and a thin knife inserted into the center of the muffins comes out clean. Transfer the pan to a rack and cool for 5 minutes before carefully removing each muffin to the cooling rack.