Preheat your oven to 400 degrees Fahrenheit. Line a large baking sheet with parchment paper and set it aside. This ensures the shortcakes won't stick.
In a large bowl, combine 2 and 1/2 cups all-purpose flour, 1/2 teaspoon salt, 1 tablespoon baking powder, 1/2 teaspoon baking soda, and 1/4 cup granulated sugar. Mix everything well until evenly combined, creating a dry mixture.
Cut 1 stick (4 ounces) unsalted butter into small cubes. Work it into the flour mixture using your fingers, two forks, or a pastry cutter until it resembles coarse meal. You should see pea-sized chunks of butter throughout.
In a separate bowl, whisk together 1 large egg, 1/2 cup buttermilk, and 2 tablespoons heavy cream. This will be our wet mixture.
Add the wet mixture to the flour/butter mixture. Use a fork to stir everything together until just moistened. Be careful not to overwork the dough; you want it to remain tender.
Turn the dough out onto a clean, lightly floured work surface. Gently knead it about 5 or 6 times to bring it together. The dough should be cohesive but not overworked.
Pat the dough into an even thickness of about 1 and 1/2 inches. Use a 3-inch biscuit cutter to cut out 4 rounds. Carefully transfer the biscuits to the prepared baking sheet, placing them 2 inches apart.
Lightly brush the top of each biscuit with some additional heavy cream. This will give them a beautiful golden color while baking.
Bake for 18 to 20 minutes, or until the biscuits are golden brown. Allow them to cool for 10 minutes on the baking sheet; this helps them firm up.
While the biscuits are cooling, heat your grill to medium-high. Brush the 4 ripe peaches with 1 tablespoon canola oil. Place them on the grill for about 4 minutes; then gently flip the peaches and continue to grill for another 3 to 4 minutes. Look for beautiful grill marks and a caramelized appearance.
Once the peaches are done, carefully remove them from the grill and set them aside until needed.
In a large bowl, using a handheld mixer or a stand mixer fitted with the whisk attachment, beat together 1 cup heavy whipping cream and 2 tablespoons confectioners' sugar until soft peaks form. Set aside or refrigerate until you're ready to serve.
To assemble, slice the biscuits in half. Layer them with the whipped cream, grilled peach slices, and a mix of 1/2 cup fresh blueberries, 1/2 cup fresh raspberries, and 1/2 cup fresh blackberries. If desired, garnish with mint springs for a pop of color and freshness.
Serve immediately and enjoy this delicious summer treat!