Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

Cream together the unsalted butter and sugar until light and fluffy, about 3-5 minutes.

Add the large eggs one at a time, ensuring each is well mixed before adding the next.

Mix in the sour cream and vanilla extract until smooth.

In another bowl, whisk together flour, baking powder, and salt. Gradually add to the wet mixture.

Fold in the rhubarb pieces into the batter.

Pour the batter into the prepared baking pan and smooth the top.

For the streusel, mix together flour, sugar, and chilled butter until crumbly.

Sprinkle the streusel topping evenly over the batter.

Bake for 40-50 minutes, or until a toothpick comes out clean.

Let cool in the pan for about 10-15 minutes before cutting.