Preheat the oven to 350°F and line an 11x7 baking sheet with parchment paper. Grease the paper and edges of the pan with nonstick cooking spray and set aside.
In a large bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.
In a separate bowl, beat together the butter and sugar. Add the sour cream and eggs and beat until smooth.
Combine the buttermilk, red food coloring, and vanilla.
Add half the flour mixture to the butter mixture and beat to combine. Add half the buttermilk mixture and beat to combine.
Mix in the remaining flour mixture until combined, then mix in the remaining buttermilk mixture.
In a small bowl, combine the baking soda and vinegar. Add to the batter and fold until just combined.
Spread the mixture over the prepared baking sheet and bake in the preheated oven until a toothpick inserted in the center comes out clean, 25 to 30 minutes.
Remove from the oven and allow to cool on a wire cooling rack until cooled completely.
When the cake is cool, make the icing by beating together the cream cheese and butter. Add the powdered sugar and half the heavy cream and beat to combine. Add more cream, ½ tablespoon at a time, to thin the icing to the desired consistency.
Add more cream, ½ tablespoon at a time, to thin the icing to the desired consistency.
Decorate with holiday sprinkles and cake crumbs, if desired.