In a large mixing bowl, cream together the shortening and brown sugar. Beat in the buttermilk, egg, and vanilla extract until smooth.
Add the flour, baking soda, and salt to the mixture, blending until just combined. Fold in the chopped rhubarb and spread the batter into a greased 9x13-inch pan.
In a small bowl, mix together the granulated sugar and cinnamon. Stir in the pecans and coconut, if using. Sprinkle this mixture evenly over the batter.
Bake at 350°F for 40-50 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool for at least 20 minutes before serving.
Enjoy with a dollop of whipped cream or a scoop of ice cream!