Preheat oven to 375 degrees F (190 degrees C). Coat top and insides of jumbo muffin pan with cooking spray, or line with muffin papers.
In a large bowl, cream together 1/2 cup butter, 1 1/4 cups sugar, vanilla, and salt until light and fluffy. Beat in eggs, one at time. Mix together the 1 3/4 cups of flour, baking powder, and 1/2 teaspoon cinnamon. Stir in the flour mixture alternately with the milk, mixing just until incorporated. Fold in apples and walnuts. Scoop into muffin cups to fill 3/4 full. Lightly sprinkle the tops with cinnamon crumble.
To make Cinnamon Crumble: In a small bowl, combine 1/4 cup white sugar, 1/2 teaspoon of cinnamon, 3 tablespoons all-purpose flour, and 2 tablespoons cubed butter. Mix together using a pastry blender, or two butter knives. The crumble topping should resemble small peas.
Bake in preheated oven for 30 minutes, or until tops are golden brown, and a toothpick inserted into the center comes out clean. If they are browning too quickly, cover with tin foil. Remove from pans, and cool on wire rack.