Pre-step: prepare red velvet cake in 9 x 13 inch pan, as instructed on the box. (Cake should be fully cooled before assembling trifle.)
- Using electric beaters or stand mixer with paddle attachment, beat softened cream cheese on medium speed until smooth, about 1 minute. Scrape down the bowl half way through if necessary to ensure all the cream cheese is smooth.
- Add sour cream and vanilla to cream cheese, beat again on medium speed until ingredients are fully incorporated and the mixture is smooth, about 30 seconds - 1 minute.
- Add powdered sugar in 1/2 cup increments, beating on medium speed in between each addition until the powdered sugar is fully incorporated and the mixture is smooth. Set aside.
- If using hand beaters, place the heavy whipping cream in a separate large bowl. If using a stand mixer, scrape out the cream cheese mixture into a separate large bowl, then place the heavy whipping cream into the stand mixer bowl and switch to the whip attachment. (No need to wash the bowl or hand beaters as long as you scrape them off.)
- Beat the heavy whipping cream, starting out on low speed for 1 minute (to avoid splatter), then increasing to high speed until stiff peaks form, about another 3 - 4 minutes.
- With a large wooden spoon or rubber spatula, stir about 1/3 of the whipped cream into the cream cheese mixture. Then, gently fold another 1/3 of the whipped cream into the cream cheese mixture. Repeat with the remaining whipped cream. Try to avoid deflating the whipped cream by over-mixing.
- If making one large trifle, cut the cake into three sections. Gently break apart one third of the cake into chunks and place in the bottom of a trifle bowl. Top with 1/3 of the cream cheese filling. Repeat the process two more times for a total of three layers, ending with the cream cheese layer.
- If making individual trifles, cut the cake into pieces matching the number of trifles you are making. Then divide each piece into three. (For example, if you are making 9 individual trifles, cut the cake into 9 pieces, then divide each of those pieces into 3.) Then follow the instructions for layering in step 7 above.