Prepare the syrup: In a heavy small saucepan, bring the sugar and 1 cup cold water to a boil over high heat, stirring with a wooden spoon until the sugar dissolves. Continue to boil, without stirring, for 5 minutes, then remove from the heat. Crush the mint leaves and add to the syrup. Cool completely, then strain and discard the mint leaves.
Prepare the shortcake: Preheat the oven to 350°F.
In a large bowl, sift together the flour, sugar, baking powder, and salt. Using your fingers, work the butter into the flour mixture until it looks crumbly.
Stir in the cream and mix just until it is absorbed and the dough is no longer sticky and releases from the sides of the bowl.
On a lightly floured surface, flatten the dough into a 1-inch flat cake. Cut into 8 equal wedges. Form each wedge into 1-inch-thick rounds. Place the shortcakes on an ungreased baking sheet. Bake for 25 to 35 minutes until golden.
Prepare the berries: Wash, hull, and slice the strawberries, then place in a bowl and add the other berries, if using. Add as much of the mint-infused syrup as you like and toss. Let stand for 10 minutes.
Using a serrated knife. cut the shortcakes in half crosswise and place them on individual serving plates. Divide the berries onto the bottom half, replace the tops, and serve with the whipped cream.