Preheat oven to 350°F (175°C).
Butter a 9x5-inch loaf pan and line it with parchment paper, leaving overhang on the sides for easy removal.
In a medium bowl, whisk together the melted butter, granulated sugar, eggs, sour cream or yogurt, lemon zest, and vanilla extract until well combined.
In a large bowl, whisk together the flour, baking powder, and salt.
Add the chopped rhubarb to the dry ingredients and toss to coat evenly.
Pour the wet mixture into the dry ingredients and stir gently until just combined—avoid overmixing.
Transfer the batter into the prepared loaf pan and spread it out evenly.
Sprinkle the remaining ¼ cup of rhubarb and a bit of sugar over the top of the batter.
Bake for 55–65 minutes, or until a toothpick inserted into the center comes out clean.
Let cool in the pan for about 10–15 minutes, then lift out using the parchment overhang and cool completely on a wire rack.