Rhubarb Streusel Cake with Warm Vanilla Sauce

Cake:
2 tablespoons butter, softened
1 cup sugar
1 egg
2 cups all-purpose flour
1 teaspoon baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
1 cup buttermilk
2 cups chopped fresh or frozen rhubarb

Streusel Topping:
1/2 cup all-purpose flour
1/2 cup sugar
4 tablespoons butter, melted

Vanilla Sauce:
1/2 cup butter
3/4 cup sugar
1/2 cup evaporated milk
1 teaspoon vanilla extract

Preheat the oven to 350 degrees. Lightly grease a 9-inch square baking dish and set aside.

In a large mixing bowl, cream together the butter and sugar for the cake. Beat in the egg. In a smaller bowl, whisk together the flour, baking powder, baking soda and salt. Add the dry ingredients to the creamed butter/sugar mixture alternately with the buttermilk, beating just until moistened. Fold in the chopped rhubarb. Pour the batter into the prepared pan and smooth the batter to the edges.

In a medium bowl, combine the streusel topping ingredients, mixing them together until the mixture is coarse and combined. Sprinkle the streusel evenly over the batter. Bake for 40-45 minutes or until a toothpick comes out clean and the top of the cake is lightly browned. Cool on a wire rack.

For the sauce, melt butter in a medium saucepan. Add the sugar and milk. Bring to a boil; cook and stir for 2-3 minutes or until slightly thickened (it will thicken more as it cools slightly). Remove from the heat; stir in the vanilla. Serve warm with the cake.