For the cream cheese swirl: Mix together the cream cheese, sugar and egg white in a small bowl until smooth.
For the berries and cream: Gently toss the mixed berries with the sugar, lemon juice and orange zest in a large bowl. Let stand, gently stirring occasionally, until the sugar has dissolved and the mixture is juicy (some of the raspberries will break down), about 1 hour. Refrigerate until ready to serve.
For the cake: Preheat the oven to 350 degrees F. Spray a 9-inch square baking dish with nonstick cooking spray and line the bottom with parchment.
Whisk together the flour, baking powder and salt in a medium bowl. In a large bowl, beat together the butter and sugar with an electric mixer until light and fluffy. Beat in the vanilla and egg. Add the flour mixture in 3 batches, alternating with the milk and beginning and ending with the flour. Beat until the flour is just incorporated.
Spoon the batter into the prepared baking dish and smooth with an offset spatula. Add big dollops of the cream cheese mixture all over the surface and use a small spoon to swirl it in slightly, then smooth the top again.
Bake the cake until a wooden toothpick inserted into the center comes out clean, about 30 minutes. Cool the cake in the baking dish for 10 minutes, then remove and cool it completely, cream cheese-side-up.
Cut the cake into squares and serve topped with the berries. Add a dollop of whipped cream. Garnish with the mint.