Preheat the Oven: Preheat your oven to 375°F (190°C). This ensures that the muffins bake evenly from the moment they go in.
Prepare the Muffin Tin: Grease a muffin tin or line with muffin liners to prevent sticking.
Mix Dry Ingredients: In a large mixing bowl, whisk together 2 cups of all-purpose flour, ¾ cup of granulated sugar, 2 teaspoons of baking powder, ½ teaspoon of baking soda, ½ teaspoon of salt, and 1 tablespoon of ground cinnamon until evenly combined.
Combine Wet Ingredients: In another bowl, mix together ¾ cup of milk, ⅓ cup of vegetable oil, and 1 large egg until well blended.
Combine Wet and Dry Mixtures: Pour the wet ingredients into the dry ingredients. Stir gently until just combined, being careful not to overmix; a few lumps are okay. Overmixing can lead to dense muffins.
Prepare the Filling: In a small bowl, combine 1 tablespoon of ground cinnamon and ¼ cup of granulated sugar to make the sweet cinnamon filling.
Fill the Muffin Tins: Spoon a tablespoon of muffin batter into each muffin cup, then sprinkle a teaspoon of the cinnamon-sugar mixture on top. Follow with another tablespoon of batter to cover the filling completely.
Create the Topping: In a small bowl, mix ¼ cup of brown sugar and 2 tablespoons of melted butter together. Stir until crumbly.
Add the Topping: Generously sprinkle the topping over each muffin before placing them in the oven.
Bake: Place the muffin tin in the preheated oven and bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Once baked, remove the muffins from the oven and allow them to cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely.