Preheat the oven to 375 degrees F (190 degrees C). Coat a 6-cup jumbo muffin tin with cooking spray or line cups with paper liners.

Beat 1 1/4 cups sugar, 1/2 butter, vanilla extract, and salt together in a large bowl with an electric mixer until light and fluffy; beat in eggs, one at a time, beating well after each addition. Whisk 1 3/4 cups flour, baking powder, and 1/2 teaspoon cinnamon together in a separate bowl; stir into sugar mixture, alternating with milk, until just combined. Fold in apples and walnuts until evenly distributed. Spoon batter into the prepared muffin cups, filling each about 3/4 full.

Combine 1/4 cup sugar, 3 tablespoons flour, 2 tablespoons butter, and 1/2 teaspoon cinnamon in a bowl using a pastry blender or 2 butter knives until crumbly; lightly sprinkle over batter.

Bake in the preheated oven until tops are golden brown and a toothpick inserted into centers comes out clean, about 30 minutes. Cover with aluminum foil if tops start to brown too quickly. Remove muffins from tin; cool on a wire rack.