Crumb topping:
1 cup all-purpose flour
1/3 cup granulated sugar
1/4 cup light brown sugar, lightly packed
1/2 teaspoon kosher salt
1/4 teaspoon ground cinnamon
1/4 pound (1 stick) cold unsalted butter, diced
Muffins:
Grated zest and juice of 1 orange
About 3/4 cup buttermilk
2 large eggs
3 Tablespoons honey
1/8 teaspoon Fiori di Sicilia extract (from King Arthur Flour, optional)
1 stick (8 Tablespoons) unsalted butter, melted and cooled
1/3 cup sugar
2 cups all-purpose flour
2-1/2 teaspoons baking powder
1/4 teaspoon baking soda
1/4 teaspoon salt
1 cup raspberries – fresh or frozen (not thawed)
1 cup blueberries – fresh or frozen (not thawed)
2/3 cup chopped walnuts
Center a rack in the oven and preheat the oven to 400 degrees F. Butter or spray the 12 molds in a regular-size muffin pan or fit the molds with paper muffin cups. Place the muffin pan on a baking sheet.
Make the crumb topping:
Combine the flour, granulated sugar, brown sugar, salt, cinnamon, and the butter crumb topping ingredients in the bowl of an electric mixer fitted with a paddle attachment. Mix on low speed until the butter is the size of peas. Rub the mixture with your fingertips until it's in big crumbles, then set aside in the refrigerator, tightly covered, until ready to use.
Make the muffins:
Pour the orange juice into a large glass measuring cup or a bowl and pour in enough buttermilk to make 1 cup. Whisk in the eggs, honey, melted butter and Fiori di Sicilia (if using).
In a large bowl, rub the sugar and orange zest together with a spatula until the sugar is moist. Whisk in the flour, baking powder, baking soda and salt. 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 bubbly, and that’s just the way it should be. Stir in the blueberries. Divide the batter evenly among the muffin cups. Sprinkle on the crumb mixture (you may have some leftover - just freeze if for another time).
Bake for 22 to 25 minutes (mine were done at 21 minutes). When fully baked, the crumb topping will be golden, the muffins springy to the touch and a thin knife inserted into the center of the muffins will come out clean. Transfer the pan to a rack and cool for 5 minutes before carefully removing each muffin from its mold.