How to Make It
Preheat oven to 400°F. Lightly grease a 12-cup muffin pan with cooking spray or line with baking liners.
Place apple and granulated sugar in a medium bowl; toss to coat, and let stand 10 minutes.
Meanwhile, whisk together flour, brown sugar, baking powder, salt, and cinnamon in a large bowl.
Stir milk, melted butter, vanilla, and eggs into apple mixture; add to flour mixture, stirring until just combined. Divide batter evenly among prepared muffin cups.
Prepare the Crumb Topping: Stir together 1/3 cup each of all-purpose flour, packed dark brown sugar, and very finely chopped walnuts, and 1/4 tsp. ground allspice in a bowl. Add 3 Tbsp. melted salted butter, and stir until crumbly. Sprinkle crumb topping evenly over muffin batter in pan.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 18 to 19 minutes. Cool in pan 5 minutes; remove muffins to a wire rack, and let cool 15 minutes to serve warm. Or let cool to room temperature.