In a medium bowl, combine the chopped rhubarb with 1/4 cup granulated sugar. Stir gently and set aside. This will help draw out some moisture and lightly sweeten the rhubarb.

In a separate small bowl, whisk together 1/2 cup flour, 1/4 cup brown sugar, 1/4 cup granulated sugar, and 1/2 tsp cinnamon. Pour in the 1/4 cup melted butter and mix with a fork until moist crumbs form. Set aside.

Preheat your oven to 350°F (175°C). Grease and flour a 9x5-inch loaf pan, or line with parchment paper leaving an overhang on the long sides for easy removal.

In a large bowl, whisk together 1 3/4 cups all-purpose flour, 1 tsp baking soda, and 1/2 tsp salt. Set aside.

In another large bowl, using an electric mixer, cream together 1/2 cup softened butter and 1 cup granulated sugar until light and fluffy (about 2-3 minutes).

Beat in the large egg until well combined, then stir in the 1 tsp vanilla extract.

Gradually add the dry ingredients to the wet ingredients, alternating with the 1/2 cup buttermilk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.

Gently fold in the sugared rhubarb, along with any juices that have accumulated, using a rubber spatula.

Pour the batter into the prepared loaf pan and spread evenly.

Evenly sprinkle the streusel topping over the top of the batter.

Bake for 55-65 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean or with moist crumbs (not wet batter). If the streusel starts to brown too quickly, you can loosely tent the top with foil.

Remove the loaf pan from the oven and let the bread cool in the pan on a wire rack for 15-20 minutes before carefully lifting it out by the parchment paper overhang (or inverting if no parchment) to cool completely on the wire rack. Slicing too early may cause it to crumble.