Preheat the oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment paper or coat it with non-stick spray.

In a medium bowl, whisk together the flour, baking soda, and salt until well combined; set aside.

In a large bowl, cream the softened butter and 1 1/2 cups sugar for 2–3 minutes until light and fluffy.

Beat in the eggs one at a time, then mix in the vanilla extract until the batter is smooth.

Add half of the dry ingredients to the wet mixture and stir just until almost combined. Mix in the sour cream, then fold in the remaining dry ingredients until no flour streaks remain.

In a small bowl, combine 1/4 cup sugar with the cinnamon. Keep the melted butter nearby.

Spread half of the batter into the prepared loaf pan. Brush with half of the melted butter and sprinkle with half of the cinnamon sugar mixture.

Add the remaining batter, smooth the top, brush with the remaining melted butter, and finish with the rest of the cinnamon sugar.

Bake for 50 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Cool in the pan for 10 minutes, then transfer to a rack to cool completely before slicing.