The rhubarb in this cake is not agressive; it adds a tartness to contrast with the sweetness of the streusel, but it's not overpowering. If you want a more assertive flavor, add more rhubarb, more lemon, or both.
1/3 cup sugar
zest from one lemon (about 2 teaspoons)
juice from 1 lemon (about 1 tablespoon)
For the topping:
3/4 cups unbleached all-purpose flour
1 tablespoon brown sugar
1 tablespoon granulated sugar
4 tablespoons butter
For the cake:
1 cup unbleached all-purpose flour
1 teaspoon baking powder
1/4 teaspoon baking soda
1/2 teaspoon salt
1/2 cup sour cream
1 teaspoon vanilla
4 tablespoons butter, softened
3/4 cup granulated sugar
Place the rack in the middle of the oven and preheat the oven to 350 degrees. Butter and flour an 8-inch square baking pan, making sure to shake out any excess flour. I usually do this by turning it upside down over my sink and gently tapping it on the top of my faucet. But then again my faucet has begun to leak, so mayber a whack with the heel of your hand would be better.
In a small bowl, stir together the rhubarb, sugar and lemon zest. Set aside.
Make the streusel topping by whisking together the flour and sugars. With your hands, work in the pieces of butter by pinching them repeatedly until they incorporate into the flour and form large clumps. Set aside.
To make the cake, whisk together the flour, baking powder, baking soda, and salt in a small bowl. In another small bowl or measuring cup, mix together the sour cream and vanilla.
Beat the butter and sugar together using a mixer at medium-high speed until it is pale and fluffy, about 5 minutes. Add one third of the flour and, with the mixer on low, combine just until the flour has blended in with the butter mixture. Add one third of the sour cream and mix briefly. Continue alternating the flour and sour cream, mixing each time just until the new ingredient has incorporated.
Spoon the mixture into a pan and smooth it evenly across the pan. With a slotted spoon, remove the rhubarb and distribute it evenly on top of the batter, leaving any remaining liquid in the bowl. Sprinkle the streusel mixture evenly over the top of the cake. If you have remaining liquid, drizzle it over the top of the streusel. Bake until a toothpick inserted in the center comes out clean and the streusel is golden brown, about 50 minutes.
Let the cake cool completely in the pan and then cut it into squares for serving.