Preheat oven to 350ºF (177ºC) and prepare parchment-lined 8½ x 4½-inch (1.5 litre) glass loaf pan.
In a large bowl, press together sugar and orange zest using a spatula or the back of a wooden spoon until the sugar takes on an orange hue and is fragrant. Whisk in egg, buttermilk, oil and vanilla until well combined.
Add flours, baking powder, baking soda, cardamom and salt and mix until combined. Do not overmix, it’s okay to still see flour remnants around the edges of the bowl. Fold in rhubarb and stir just until it’s distributed evenly through the batter.
Turn batter into prepared pan, smoothing out the top with a spatula if needed.
Sprinkle remaining sugar across the top, then bake for 60-65 minutes until a tester inserted into the centre comes out clean. *If using a metal loaf pan, don’t forget to tent your loaf with foil to help prevent the top and outer edges from over-baking.
Set loaf on a wire cooling rack and let it cool in the pan for 15 minutes. Using parchment edges, lift loaf from pan to rest on rack until cooled completely (about 1 hour).