2cups plus 2 tablespoons ; about 280gall-purpose flour
½teaspoonfine sea saltbeaten
8 ounces; 250gunsalted butterat room temperature
½cupunsweetened shredded coconut
Preheat the oven to 350F (180C). Lightly butter a 7 x 12-inch (17-1/2 x 30cm) baking pan.
Place the rhubarb and ¼ cup of the sugar in a small bowl, stir, and set aside while you prepare the cake.
Sift together the dry ingredients onto a piece of parchment paper.
In a large bowl, or the bowl of an electric mixer, mix the butter with the remaining 1-3/4 cups sugar until pale yellow and light. Add the egg and the vanilla and mix well.
Add the dry ingredients in thirds to the butter mixture, alternating with the buttermilk, beginning and ending with the dry ingredients. Fold in the rhubarb mixture, and then the coconut. Spoon the batter, which will be fairly thick, into the prepared baking dish.
Bake until the cake is golden and puffed, and your finger leaves a slight indentation when you press on the top, about 45 minutes.
Remove the pan from the oven and set it on a wire rack to cool. Cut and serve as you like.