In a large mixing bowl beat butter and sugar on medium speed for about 5 minutes, until fluffy. Add eggs one at a time, and blend well.
Reduce to low speed, and add flour slowly. Mix in baking powder and salt.
Using 4 medium sized mixing bowls, and one flavored milk to each bowl.
Pour cake batter equally into each of the 4 bowls. Stir to combine with milk.
Add 1-2 drops of food coloring to each bowl. Yellow for egg nog, red for strawberry créme, and orange for orange créme. Add cocoa to chocolate marshmallow batter.
Grease four 6” x 6” cake pans with butter and lightly flour.
Pour each bowl of batter into its own baking pan.
Bake for 20-25 minutes until a toothpick inserted in the center of cake comes out clean.
Remove from oven and let cool in pans for 10 minutes.
Carefully remove cakes from pan and cool completely on a wire rack.
For Frosting: In a mixing bowl, beat butter on high speed until fluffy. Reduce to medium and slowly add powdered sugar and egg nog until combined.
Use a round cookie cutter that fits within the mouth of a mason jar. about 3”.
Cut circles out of each cake flavor. Stack each cake circle layered with icing.
Drop the stacks of cake into each mason jar.
Using an icing/piping bag with a decorator tip of your choice, swirl frosting on top. Garnish with rainbow sprinkles and serve immediately.