In a medium bowl, combine the pitted and halved cherries, granulated sugar, cornstarch, and lemon juice. Gently toss to coat the cherries evenly. Set aside to allow the mixture to macerate, which will enhance the flavor and create a syrup.

Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it with non-stick spray.

In a large mixing bowl, whisk together the sifted flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon. This ensures even distribution of the leavening agents and spices.

In another bowl, whisk together the buttermilk, melted butter, eggs, and vanilla extract until well combined. Mixing wet ingredients separately helps to incorporate them smoothly into the dry ingredients.

Pour the wet ingredients into the dry ingredients and gently fold them together using a spatula until just combined. Be careful not to overmix; a few lumps are okay to keep the muffins tender.

Gently fold the cherry filling into the muffin batter, ensuring an even distribution without breaking up the cherries too much.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.

In a small bowl, mix together the brown sugar, oats, melted butter, and ground cinnamon until crumbly. This will create a sweet, crunchy topping.

Sprinkle the topping evenly over each muffin.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean. The muffins should be golden brown on top.

Once baked, remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a cooling rack to cool completely.