Preheat and prep the pan: Heat oven according to your cake mix directions (usually 350°F). Grease a 9x13-inch baking pan or line it with parchment for easy lifting.
Make the cake batter: Prepare the red velvet cake mix as directed.
For scratch: whisk flour, sugar, baking soda, baking powder, salt, and cocoa. In another bowl, combine buttermilk, eggs, oil, vanilla, vinegar, and food coloring. Stir wet into dry until smooth.
Bake: Pour batter into the pan and bake 25–35 minutes, or until a toothpick inserted in the center comes out clean.
Do not overbake—this cake should be soft.
Poke the holes: Let the cake cool 10 minutes. Using the handle of a wooden spoon or a thick straw, poke holes all over the cake about 1 inch apart. Go almost to the bottom but not through the pan.
Mix the pudding: In a bowl, whisk the instant pudding with 2 cups cold milk for about 2 minutes until slightly thickened but still pourable.
Soak the cake: Pour the sweetened condensed milk evenly over the warm cake, letting it seep into the holes.
Then pour or spoon the pudding over the top, nudging it into the holes with a spatula. Cool to room temperature, then refrigerate for at least 2 hours to set.
Make the topping: Beat the softened cream cheese, powdered sugar, and vanilla until smooth. In a separate bowl, whip the cold heavy cream to stiff peaks.
Fold the whipped cream into the cream cheese mixture until fluffy.
Frost and decorate: Spread the topping over the chilled cake in smooth swoops. Add sprinkles, crushed candy canes, or white chocolate shavings for a festive finish.
Chill and serve: Chill at least 30 minutes before slicing for clean cuts. Serve cold or slightly cool.