Preheat the oven to 425 degrees Fahrenheit and line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Add the cold cubed butter and cut it into the flour mixture with a pastry cutter or your fingertips until the mixture resembles coarse crumbs.
Stir in the buttermilk, lemon zest, fresh lemon juice, and vanilla extract just until a soft dough forms. Do not overmix.
Turn the dough onto a lightly floured surface and gently pat it to about 1-inch thick.
Cut the dough into 6 rounds using a biscuit cutter and place them on the prepared baking sheet.
Bake for 16 to 18 minutes, or until the tops are lightly golden. Transfer to a cooling rack and let cool slightly.
While the shortcakes bake, combine the sliced strawberries, granulated sugar, and lemon juice in a medium bowl. Stir well and let the berries sit for at least 15 minutes to release their juices.
In a separate bowl, beat the heavy whipping cream, powdered sugar, and vanilla extract until soft peaks form.
To assemble, split each warm shortcake in half. Spoon strawberries and their juices over the bottom half, add a generous dollop of whipped cream, then place the top half over the filling.
Spoon a little extra strawberry-lemon syrup over the top and garnish with fresh lemon slices, if desired. Serve immediately.