Preheat oven to 350º.
Line bottom of a 10" x 15" x 1" rimmed baking sheet with parchment paper (make sure that it lays flat on the bottom). Do not grease the pan.
Prepare cake mix according to package directions. Pour into prepared pan and bake at 350º for 20 minutes, until the top is golden and the edges start to crack. *NOTE: it will rise up in the pan significantly, even over the top. But it should not flow over.
Loosen cake from the edges of the pan while it's still warm. It's going to be a little sticky, like angel food cake is, but it will come away easily.
Spread out a clean kitchen towel and sprinkle evenly with 1/4 cup powdered sugar. Flip warm cake out onto the towel, peel off the parchment paper, and immediately roll up, starting at the short end, WITH the towel inside. Let cool completely (at room temperature for 1-2 hours or pop it in the refrigerator for an hour.
In a large bowl, beat the cream cheese with a mixer until smooth. Add 3/4 cup powdered sugar, vanilla, and cream and beat on low until combined, then on high for 3-4 minutes until fluffy and stiff peaks form.
Unroll cooled cake and spread with half of the filling and top with half of the berries. Carefully roll back up (this time WITHOUT the towel) and place on serving plate. Spread top and sides with remaining filling and top with remaining berries.
Serve immediately. Leftovers are best eaten within 24 hours.