Have all ingredients at room temperature.

Preheat oven to 350°F with a rack in the center, and grease a 9 x 13 x 2-inch baking pan.

In a medium mixing bowl, stir the rhubarb pieces together with two tablespoons of the sugar. Let sit while you make the batter.

In another medium mixing bowl, stir together the flour, baking powder, baking soda, salt, and cardamom.

In a small bowl, stir together the sour cream and vanilla extract.

In a large bowl (or the bowl of a. stand mixer), beat together the butter and the remaining cup sugar until lightened in color and texture, about 3 minutes.

Add the eggs one at a time, beating to incorporate after each addition.

Then add the flour mixture in three parts, alternating with the sour cream in two parts and beating on low after each addition until just incorporated.

Gently fold the rhubarb into the batter.

Scrape the batter evenly into the prepared pan.

To make the streusel topping, stir together the flour, brown sugar, nuts, cinnamon, and salt in a small bowl.

Pour the melted butter overtop and stir to incorporate evenly into the dry ingredients.

Use your fingers to sprinkle the topping over the batter.

Bake the cake on the center rack until a tester inserted into the middle of the cake comes out almost clean, about 40 minutes.

Cool in the pan for at least 10 minutes before serving warm or at room temperature.