Muffins:
2 cups all-purpose flour
2/3 cup sugar
2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1/4 teaspoon ground cinnamon
2 large eggs
1 stick unsalted butter, melted
1 cup buttermilk
1 tablespoon grated lemon zest
3/4 cup fresh or frozen blueberries
3/4 cup fresh or frozen raspberries
Topping:
1/4 cup all-purpose flour
2 tablespoons butter, softened
2 tablespoons sugar
2 tablespoons finely chopped walnuts
1 teaspoon finely grated lemon peel
Preheat the oven to 400 degrees F. Line a standard 12-cup muffin tin with paper liners.
First make the topping: in a medium bowl, mix together the topping ingredients until crumbly. Set aside.
In a bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon.
In a large bowl, beat the eggs with the melted butter. Add the buttermilk and lemon zest and whisk to combine. Add the dry ingredients to the wet and stir just to combine. (Batter will still be somewhat lumpy; do not overmix.) Gently fold in the blueberries and raspberries so the berries do not break open. Using a standard cookie scoop, scoop mixture into the prepared muffin tin. Crumble the streusel topping over the muffins. Bake muffins until golden brown and a toothpick inserted into the center comes out clean, about 20 minutes.
Let cool in the tins for 10 minutes, then remove and cool on wire racks.