Preheat your oven to 350°F (175°C) and grease a 13×9 inch baking pan with butter or non-stick spray.

In a large mixing bowl, combine ½ cup softened butter, ¾ cup brown sugar, and ½ cup granulated sugar. Beat on medium speed until light and fluffy, about 3-5 minutes.

Add 1 egg and 1 teaspoon of vanilla extract to the mixture and mix until well combined.

In a separate bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking soda, and ½ teaspoon salt. Gradually mix into the wet ingredients.

Alternately add 1 cup of buttermilk and the dry mixture to the batter, stirring gently until combined.

Fold in 2 cups of chopped rhubarb carefully to ensure even distribution.

Pour the batter into the prepared baking pan and spread evenly.

Mix together ½ cup of sugar, 3 tablespoons of cold butter, ¼ teaspoon of cinnamon, ½ cup of shredded coconut, and ½ cup of chopped pecans until crumbly.

Sprinkle the crumble evenly over the batter in the pan.

Bake the cake in the preheated oven for 45–50 minutes, checking for doneness with a toothpick.

Let the cake cool in the pan for about 30 minutes before serving.