Preheat oven to 350°F (175°C). Grease or line a 9x5-inch loaf pan with parchment.

In a medium bowl, whisk flour, baking soda, baking powder, salt, and cinnamon.

In a large bowl, cream butter and sugar until light and fluffy.

Beat in eggs one at a time. Add vanilla and sour cream. Mix until smooth.

Gently stir in dry ingredients until just combined.

Fold in rhubarb and orange zest (if using).

Pour batter into the prepared pan and smooth the top.

Bake for 55–65 minutes, or until a toothpick inserted comes out clean.

Cool in pan for 10 minutes, then transfer to a wire rack to cool completely.