Preheat oven to 350°F. Grease two 8x4-inch loaf pans (or line with parchment for easier removal).
In a large bowl, whisk together brown sugar and oil until smooth. Add egg, then stir in buttermilk* and vanilla.
In another bowl, whisk together flour, baking soda, and salt. Gently fold into the wet mixture until just combined (don’t overmix).
Fold in chopped rhubarb and walnuts (if using).
Make streusel – In a small bowl, mix flour, sugar, softened butter, and cinnamon with a fork until crumbly.
Divide half the batter between the two pans. Sprinkle half the streusel over. Repeat with the remaining batter and streusel.
Bake 60–65 minutes, until a toothpick inserted in the center comes out clean.
Let loaves cool in pans for 10–15 minutes. Remove to a rack, slice with a serrated knife, and enjoy plain or with butter/jam.