Preheat your oven to 400°F (200°C) and line a 12-cup muffin tin with paper liners.

In a large bowl, whisk together the flour, sugar, baking soda, baking powder, and salt until evenly blended.

In another bowl, combine the beaten egg, cranberry sauce, milk, oil, and vanilla; whisk until smooth.

Pour the wet mixture into the dry ingredients and stir until just combined, being careful not to overmix.

In a small bowl, stir together cranberry sauce, sugar, and softened butter until evenly mixed.

Divide half the batter among the muffin cups, top each with about 1 teaspoon of the swirl mixture, then spoon the remaining batter on top and swirl gently with a toothpick.

Bake for 16–18 minutes until the tops spring back and a toothpick inserted comes out clean.

Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before storing.