Directions:First preheat the oven to 350 degrees Fahrenheit and grease a 9 x 9 or 8 x 8 baking pan with butter or shortening. In a large electric mixing bowl, combine the 2 tbsp. of room temperature butter with 1 cup white sugar until fluffy. Once beat well, add an egg, continuing to mix well. In a separate mixing bowl, combine 2 cups flour, baking soda, baking powder and salt. Alternate adding these dry ingredients to the other mixing bowl. Alternate the dry ingredients with buttermilk until all has been added. Mix well. Now fold the sliced rhubarb into this mixture until just combined into the batter. For the topping, get out a bowl and mix 2 tbsp. melted canna butter with 1/4 cup flour and 1/4 cup sugar. Sprinkle this over the cake batter. Bake this for 35 to 40 minutes in the preheated oven. Allow to cool out of the pan on a wire rack for 10 minutes. In a saucepan placed over medium heat mix 1/2 cup butter, heavy cream, brown sugar and vanilla. Bring to a boil for 1 minute. Remove from heat and serve over the cake. Enjoy!