Preheat oven to 425°F. In a food processor, combine flour, sugar, baking powder, lemon zest and salt and pulse to combine.
If you're not using a food processor, whisk to combine in a large bowl.
Add the butter to the food processor and quickly pulse a few times until a coarse crumb forms. Pour ⅓ heavy cream into food processor and pulse only until just combined - you don't want to overwork the dough.
If you're not using a food processor, use a pastry cutter or fork to cut the butter into the dry ingredients. Then pour heavy cream into bowl and stir until combined.
The dough is going to be crumbly and that's how it should be! Turn the dough out onto a large silicon mat or other clean surface and shape into a ball. If you find it's not quite coming together after kneading, add a teeny bit more heavy cream and knead to combine. Flatten into a ½" high disc.
Use a 3" biscuit cutter or the top of a drinking glass to cut out 4 circles (re-rolling the dough as needed). Press straight up and down with the cutter - don't twist.
Place the biscuits on a baking sheet lined with a nonstick silicone mat, making sure the insides of biscuit are touching.
Use a pastry brush to coat the tops of each biscuit with the remaining 2 teaspoons heavy cream. Sprinkle with coarse sugar, if using.
Bake for 15 minutes or until the biscuits are golden brown. Let cool on the baking sheet while you make the whipped cream.