To prepare the streusel topping, in a small bowl, stir together the brown sugar, granulated sugar and cinnamon. Set aside.

To prepare the muffins, preheat the oven to 350°F. Lightly coat a 12-cup muffin pan with nonstick cooking spray.

In a large bowl, using a handheld whisk, beat the eggs. Add the sour cream and melted butter and beat until well blended. 

In a separate bowl, sift together the flour, sugar, baking powder, cinnamon, allspice, baking soda and salt. Add to the bowl with the egg mixture and stir with a wooden spoon until most of the flour is incorporated (do not overmix).

Add the apples and walnuts and stir until the ingredients are just barely blended. 

Divide the batter evenly among the prepared muffin cups. Sprinkle the streusel topping evenly over the muffins. 

Bake until the tops of the muffins are golden brown and firm to the touch, and a toothpick inserted into the center of a muffin comes out clean, 20 to 25 minutes. 

Transfer the pan to a wire rack and let cool for 5 minutes, then run a table knife around the edge of each muffin and transfer the muffins to the rack. Serve warm or at room temperature.