Preheat your oven to 350°F (175°C) and grease a 9×5-inch loaf pan.

In a medium mixing bowl, whisk together the flour, baking soda, salt, and cinnamon.

In a separate large bowl, blend melted butter with granulated and brown sugars. Mix in the eggs, buttermilk, and vanilla extract until smooth.

Gently combine the dry mixture into the wet mixture until just incorporated.

Fold in the diced rhubarb, being cautious not to crush it.

Pour the batter into the prepared loaf pan and smooth the top.

Bake for 55-65 minutes until the top is golden brown and a toothpick comes out clean.

Allow to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.