Step 1

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

Step 2

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.

Step 3

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.

Step 4

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.