Preheat your oven to 375°F (190°C) and prepare a muffin tin by lining it with paper liners or greasing it well.
In a large mixing bowl, combine the flour, granulated sugar, baking powder, and salt, whisking them together.
In another bowl, whisk together the egg, milk, melted butter, and vanilla extract until the mixture is smooth.
Carefully fold the wet ingredients into the dry ingredients until just combined, then gently fold in the pitted cherries.
In a small bowl, combine the brown sugar, flour, and ground cinnamon. Cut in the softened butter until the mixture resembles coarse crumbs.
Spoon the muffin batter into the prepared muffin cups, filling each about three-quarters full, and then sprinkle the streusel topping over each muffin.
Bake the muffins for 20–25 minutes, or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
