Preheat the oven to 400 degrees F. Spray a 9x13 inch (or larger) sheet pan with nonstick baking spray.
In a large bowl, mix together the flour, sugar, baking powder, baking soda and salt. Add the butter and stir, then use a pastry blender, fork or your fingers to incorporate the butter until it’s in small pieces throughout the flour.
Stir in the milk, heavy cream and vanilla extract. The mixture should be thick but still wet. Stir in the strawberries and blueberries.
Use a ¼ cup measure or large ice cream scoop to scoop the shortcake batter into rounds and place it on the sheet pan. I like to do 5 or 6 rows of 3 scoops. You want them to be touching and close. Drizzle with the melted butter and sprinkle with the coarse sugar.
Bake for 28 to 30 minutes, until golden and set. Remove and let cool completely.
While the cake is cooling, make the whipped cream. Beat the cold heavy cream in the bowl of your electric mixer until just before soft peaks form. Beat in the mascarpone cheese and vanilla extract. Beat until soft peaks form.
Spread the whipped cream over the shortcake. Top with fresh berries - strawberries, blueberries, raspberries and blackberries. Sprinkle on fresh mint leaves. Slice and serve!