Allspice Crumb Muffin

 For the Streusel: 
1/2 cup all-purpose flour 
1/2 cup (packed) light brown sugar 
1/2 teaspoon ground allspice 
5 Tablespoons cold unsalted butter, cut into bits 

For the Muffins: 
2 cups all-purpose flour 
1/2 cup sugar 
1 Tablespoon baking powder 
1/2 teaspoon ground allspice 
1/4 teaspoon salt 
1/4 cup (packed) light brown sugar 
1 stick (8 Tablespoons) unsalted butter, melted and cooled 
2 large eggs 
3/4 cup whole milk 
1/4 teaspoon pure vanilla extract 
Grated zest of 1 lemon (optional) 

Getting ready: Center a rack in the oven and preheat the oven to 375 degrees F. Butter or spray the 12 molds in a regular-size muffin pan or fit the molds with paper cups. Alternatively, use a silicone muffin pan which needs neither greasing nor paper cups. Place the muffin pan on a baking sheet. 

To Make the Streusel: Put the flour, brown sugar and allspice in a small bowl and sift them through your fingers to blend. Add the bits of cold butter and toss to get irregularly shaped crumbs. Set aside in the refrigerator for the moment. (You can make the crumbs up to 3 days ahead and keep them covered in the refrigerator.) 

To 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. In a large glass measuring cup or another bowl, whisk the melted butter, eggs, milk, and vanilla extract together until well combined. Pour the liquid ingredients over the dry ingredients and, with the whisk or a rubber spatula, gently but quickly stir to blend. Don't worry about being thorough--the batter will be lumpy, and that is just the way it should be. Stir in the lemon zest, if you're using it. 

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 from its mold.