Preheat oven to 350°F. Grease a 13x9 inch baking pan with cooking spray.

Place the brown sugar, granulated sugar, and butter in a large bowl. Mix on high with a hand mixer until light and fluffy. Add the egg and vanilla, and mix to combine.

In another large bowl, whisk together the flour, baking soda, and salt.

Add half of the buttermilk to the butter/sugar mixture and mix to combine. Pour half of the flour mixture in, and stir with a spatula to combine. Repeat with the remaining buttermilk and flour mixture. Do not over mix the batter! Fold in the chopped rhubarb.

Spread the mixture into the greased 13x9 inch pan.

Cut the 3 tablespoons of butter into small cubes. Place it in a medium bowl along with the sugar. Use your fingers to rub the butter into the sugar until the butter is in pea-size pieces. Stir in the cinnamon, coconut, and pecans. Scatter the mixture over the cake batter.

Bake for 45–50 minutes, or until a toothpick comes out clean. Let cool and serve with whipped cream or Cool Whip.