• Heat the oven to 375°F and line a 12-well muffin tin with paper liners.
• In a mixing bowl with an electric mixer, beat the butter and brown sugar until light and fluffy. Beat in the eggs until well blended. Beat in the vanilla.
  1/2 cup (113 grams) unsalted butter | 3/4 cup (148 grams) packed brown sugar | 2 large eggs | 1 1/2 tsp vanilla extract
• In a separate bowl, combine the flour with the baking powder, cinnamon, and salt; stir or whisk to blend the dry ingredients.
  2 1/4 cups (284 grams) all-purpose flour | 2 1/2 tsp baking powder | 1 1/2 tsp ground cinnamon | 1/2 tsp salt
• Add one-third of the dry ingredients to the butter and brown sugar mixture and beat just until blended. Add half of the milk and beat just until blended. Repeat with another one-third of the dry ingredients and the remaining milk, then beat in the remaining flour until blended. Avoid overbeating. Fold in the chopped apple and walnuts (if using).
  2/3 cup milk | 1 cup finely chopped apple | 1/2 cup chopped walnuts (optional)
• Spoon the batter into the muffin cups, filling about 3/4 full.

Make the Topping and Bake the Muffins

• In a small bowl, combine the flour, brown sugar, cinnamon, and a pinch of salt. Work the butter into the mixture with a fork or your fingers until the mixture forms crumbs. Sprinkle the crumbs over each filled muffin cup.
  1/3 cup (43 grams) all-purpose flour | 1/3 cup (65 grams) packed brown sugar | 1/2 tsp cinnamon | pinch fine salt | 3 tbsp (42 grams) unsalted butter
• Transfer the pan to the oven and bake the apple muffins for 18 to 22 minutes or until a toothpick inserted into the center of a muffin comes out clean. Cool the muffins for a few minutes in the pan before moving them to a rack.