1. In a large bowl, combine the flour, sugar, baking powder and salt. Stir thoroughly with a wooden spoon to combine.
2. Add the pieces of cold butter and stir to coat the small pieces with flour so that they do not stick together.
3. Add the buttermilk and cream. Stir briefly to combine, leaving the mixture as lumpy as possible.
4. Pour the lumpy dough onto a lightly floured surface. Flatten gently with a rolling pin into a squarish shape approximately 1" thick.
5. Cut into 4 equal squares. Place the squares on a baking sheet and brush with melted butter. Put the entire baking sheet into the refrigerator for 20 minutes while you preheat the oven to 375°F.
6. Bake the chilled shortcakes until nicely browned, about 20 to 25 minutes.
For the topping:
1. In a saucepan, bring the water, sugar and rhubarb to a low boil. Cook, uncovered, until the rhubarb falls apart and the mixture thickens slightly to about the consistency of applesauce. Let cool.
2. Place the cream, confectioner's sugar and vanilla in a bowl and whisk until soft peaks form.
3. Putting it together: Split each biscuit in half. Spread one side with the whipped cream. Top the cream with strawberries. Spoon 1 tablespoon of the rhubarb mixture on top of the strawberries. Top with the other half of the biscuit.