Preheat the oven to 375°F (190°C). Grease the muffin tin or line with paper liners.
In a large mixing bowl, stir together 2¼ cups flour, baking soda, and salt.
In a small bowl, whisk egg, buttermilk, ½ cup melted butter, vanilla, and 1½ cups brown sugar until the sugar dissolves.
Pour the wet mixture into the dry mixture.
Add diced apples and stir gently until blended.
Spoon the batter into the muffin tin, filling each cup to the top.
In a small bowl, stir together ½ cup brown sugar, ⅓ cup flour, and cinnamon.
Drizzle in 2 tablespoons melted butter and toss with a fork until crumbly.
Sprinkle the topping evenly over the muffins.
Bake for 25 minutes or until the tops spring back when lightly pressed.
Notes
Tips and Tricks
Don’t Overmix
Stir the batter only until blended to keep the muffins light and soft.
Use Fresh Apples
Fresh, firm apples work best since they hold their shape while baking.
Check Early
Start checking at 20 minutes to avoid overbaking, since oven times can vary.
Ingredient Substitutions
Buttermilk
Use 1 cup milk mixed with 1 tablespoon lemon juice or vinegar as a substitute.
Apples
Pears can be used instead of apples for a slightly different flavor.
Brown Sugar
White sugar with 1 tablespoon molasses can replace packed brown sugar.
How to Store and Reheat
Store muffins in an airtight container at room temperature for up to 3 days. For longer storage, freeze them for up to 2 months. Reheat in the microwave for 15–20 seconds or in the oven at 300°F until warm.