In a large bowl, beat the butter and sugar until light and fluffy.
1/3 cup (75g) unsalted butter, 2/3 cup (100g) granulated sugar
Mix in the eggs, one at a time, then vanilla.
2 large eggs, 1 tsp vanilla
Stir in half of the flour mixture, followed by the sour cream, and then the remaining flour mixture until just combined.
1/2 cup (115g) full-fat or light sour cream
Transfer the batter to the prepared pan and spread it out evenly with an offset spatula. Top with the rhubarb and sprinkle evenly with streusel.
2 cups (250g) rhubarb
Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.
Notes
This cake can be stored in an airtight container or wrapped in saran and stored in the fridge for up to 3 days. Best served the day of or the day after.