Preheat the oven to 350 degrees. Line two jelly roll pans with sides about 11x17 in size with foil or parchment. Spray with a light layer of non-stick spray, set aside.

In the bowl of a stand mixer, combine the red velvet cake mix, water, eggs and oil. Start mixing on the lowest speed, working up to medium speed. Mix until thoroughly combined - stopping the machine and scraping the sides often.

Pour approximately half of the cake batter into each of the prepared baking sheets. Use a spatula to help spread the batter into the corners and as even as possible.

Bake in the preheated oven for 10 minutes. The edges should be set and the center no longer glossy. Remove from the oven and allow to cool completely to room temperature on a wire rack.

In the bowl of a stand mixer, combine the softened butter, cream cheese (if using), powdered sugar, salt, vanilla and heavy cream. Mix on low speed until combined. Bump up the speed to medium and mix until the frosting is smooth and fluffy looking. Scrape down the sides often and mix until cohesive.

Transfer the cream cheese frosting to a piping bag fitted with a large star tip.

Once the cake is cool, use a round cookie cutter or the mason jar lid to cut out the rounds of cake. Try to space them close together to get as many as possible. You should end up with approximately 30-36 rounds depending on the size of your container lid and the size of your pan. Set them aside.

Prepare your workspace with the bagged frosting, cake rounds, sprinkles and the containers with the lids off. Carefully place one round slice of cake into the bottom of the jar. You may need to fold in half to fit through the opening but we know it will fit once inside because we used the lid to cut it just right.

Follow up with a small dollop of frosting. Repeat with another layer of cake and dollop of frosting. Repeat with another layer of cake and a dollop of frosting, decorate with some sprinkles on top of the frosting.

Close the top of the jar for storage.