In a medium bowl, combine sliced strawberries, blueberries, sugar or honey, lemon juice, and lemon zest if using.Gently stir and let sit for 20–30 minutes while you prepare the shortcakes. This helps the berries release their juices and creates a naturally sweet syrup.
Preheat oven to 425°F. Line a baking sheet with parchment paper.
In a large bowl, whisk together flour, sugar, baking powder, and salt.Add cold cubed butter and use a pastry cutter, fork, or your fingers to work it into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter.Stir vanilla into the buttermilk, then pour into the flour mixture. Gently stir just until the dough comes together.
Turn the dough onto a lightly floured surface and gently pat it into a 1-inch thick rectangle.Use a round biscuit cutter or glass to cut out individual shortcakes. Press straight down without twisting.Place shortcakes on the prepared baking sheet. Brush tops lightly with buttermilk and sprinkle with coarse sugar if desired.
Bake for 12–15 minutes, or until the shortcakes are golden brown and cooked through.Transfer to a wire rack and cool slightly.
In a chilled bowl, beat heavy whipping cream, powdered sugar, and vanilla until soft peaks form.
Slice each shortcake in half horizontally.Spoon berries and their juices over the bottom half. Add a generous dollop of whipped cream, then place the top half of the shortcake over the cream.Add more berries and whipped cream on top if desired. Serve immediately.