Struesal Topped Rhubarb Cake with Vanilla Sauce

I am not much of a cake person.  I usually prefer cookies or brownies when it comes to baked desserts.  This cake, though...I couldn't stop eating it.  It is moist and has the perfect amount of tart rhubarb.  The vanilla sauce is irresistible.
I made half the batter into 12 muffins (filled halfway) and baked the other half in a 9" pie pan.  You can do it the same way or make 24 cupcakes, 2 pie pans, or a single 9" square baking dish.

Cake:
2 TBSP butter, softened
1 C sugar
1 egg
2 C flour
1 tsp baking powder
1/2 tsp baking soda
1/2 tsp salt
1 C buttermilk
2-2 1/2 C chopped (as small or large as you want) rhubarb

Streusel:
1/4 C flour
1/2 C sugar
1/4 C oats
4 TBSP butter, melted

Vanilla Sauce:
1/2 C butter
3/4 C sugar
1/2 C evaporated milk
1 tsp vanilla extract

Preheat the oven to 350.
Lightly grease your chosen baking dish.
Cream together the butter and sugar for the cake.
Add the eggs.
Put the flour, baking powder, baking soda, and salt in the bowl.
Mix while slowly pouring in the buttermilk.
Fold in the rhubarb.
Pour the batter into the prepared pans.
In a separate bowl, mix together the streusel ingredients until it forms into coarse crumbs.
Sprinkle the streusel evenly over the batter.
Bake for 20-30 minutes (depending on which type of pan you use) until a toothpick comes out clean.
To make the sauce, begin by melting the buterin a medium saucepan on medium heat.
Whisk in the sugar and milk and bring it to a boil.
Boil and stir the sauce for 2-3 minutes (it will thicken more when it begins to cool).
Remove it from the heat and stir in the vanilla.
Serve it warm over the cake.

This recipe is slightly adapted from www.melskitchencafe.com.