Macerate the strawberries: Toss the sliced strawberries with sugar and lemon juice. Let them sit at room temperature for 30 minutes to release their juices.
Prepare the dough: Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper. Whisk together the flour, sugar, baking powder, baking soda, and salt. Cut in the cold butter until the mixture has pea-sized pieces of butter throughout.
Add the buttermilk: Stir in the cold buttermilk and vanilla until a shaggy dough forms. Don't overmix.
Shape the biscuits: Turn the dough onto a lightly floured surface. Fold it over itself 3 to 4 times, then pat it to 1-inch thickness. Cut 6 biscuits with a 2.5-inch round cutter, pressing straight down without twisting.
Bake: Place the biscuits on the prepared baking sheet. Brush the tops with 1 tablespoon of buttermilk and sprinkle with coarse sugar. Bake for 12 to 15 minutes, until golden brown. Cool for 15 minutes.
Whip the cream: Beat the cold heavy cream, powdered sugar, and vanilla until medium-soft peaks form.
Assemble: Slice the biscuits in half. Spoon strawberries and some of their syrup over the bottom halves, then add whipped cream and top with the remaining biscuit halves. Serve immediately.
