For cake: Heat oven to 350 degrees F. Grease an 8-inch square pan; set aside. In bowl of electric mixer blend together sugar, egg, and butter. Beat in buttermilk until smooth. Stir together dry ingredients and add to mixer. Mix well. Stir in rhubarb. Pour batter into prepared baking pan
For Topping: Combine the topping ingredients and sprinkle over top of batter. Bake for 35 to 38 minutes or until a pick inserted in the center comes out clean. Remove from oven and place on wire rack to cool.
For sauce: Place the butter, milk, and sugar in a small saucepan over medium heat. Bring to a boil and boil for 1 minute, stirring constantly. Remove from heat and stir in vanilla. (Note: The sauce can be drizzled over the cake after it has cooled, or you can make the sauce ahead of time and refrigerate until ready to serve.)
To serve, cut into squares and place on dessert dishes. Drizzle each cake piece with vanilla sauce.
Recipe by The Culinary Cellar at https://theculinarycellar.com/buttermilk-rhubarb-cake-with-vanilla-sauce/