Preheat oven to 350ºF (177ºC) and prepare buttered or parchment-lined 8-inch baking dish.
In a medium-sized mixing bowl, stir together flour, baking powder and salt. Fold rhubarb pieces into flour mixture just until they’re flour-covered and well distributed. Set aside.
In a large bowl, cream together butter and sugar. Whisk in egg and vanilla.
Add half the flour and rhubarb mixture to the wet ingredients and stir. Pour in butter milk and continue to mix until well combined. Stir in remaining flour and rhubarb and mix just until the batter is nice and thick and the rhubarb is evenly distributed, then add the shredded coconut and stir one last time.
Spread batter into prepared pan and sprinkle the remaining teaspoon of sugar across the top. Bake for 45 minutes on the middle rack until golden and a tester comes out clean.
Remove from oven and let cool on a wire cooling rack for at least 20 minutes before slicing.