1. Preheat your oven to 350 F (175 C). Lightly grease and flour a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the sides.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt.

3. In a separate medium bowl, whisk together the egg, milk, 1/4 cup melted butter, vanilla extract, and Greek yogurt until smooth.

4. Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Do not overmix.

5. In a small bowl, combine the brown sugar and ground cinnamon. Stir well. Then, pour in the 2 tablespoons of melted butter and mix until it forms a thick, crumbly paste.

6. Pour about half of the batter into the prepared loaf pan and spread it evenly. Sprinkle half of the cinnamon swirl mixture over the batter. Carefully spoon the remaining batter over the cinnamon layer, spreading it gently to cover. Top with the remaining cinnamon swirl mixture.

7. Using a knife or a skewer, gently swirl the cinnamon mixture into the batter with a few passes. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

8. Remove the loaf pan from the oven and let it cool in the pan for about 10-15 minutes before transferring it to a wire rack to cool completely.