Step 1

Preheat oven to 350°. In a bowl, cream butter and 1 cup sugar. Add egg; beat well. Combine buttermilk and vanilla; set aside.

Step 2

Combine flour, baking soda and salt; add alternately with buttermilk and vanilla to the creamed mixture. Stir in rhubarb. Spread in a greased 13x9-in. baking pan.

Step 3

Combine the remaining sugar with cinnamon; sprinkle over batter. Bake until a toothpick inserted in center comes out clean, about 35 minutes.

Step 4

For topping, combine all ingredients; pour over individual squares.