Bake the vanilla cake according to your recipe instructions and allow it to cool completely.
Make the buttercream frosting by beating the softened butter until smooth. Add powdered sugar gradually, then mix in vanilla, salt, and milk or cream until fluffy.
Cut the cooled cake into small circles or crumble it into bite-sized pieces.
Add a layer of cake to the bottom of each mason jar.
Pipe or spoon a layer of buttercream frosting over the cake.
Repeat with another layer of cake and frosting until the jars are filled.
Finish each jar with a thick layer of buttercream on top.
Cover the top with rainbow dot sprinkles.
Serve immediately or cover and refrigerate until ready to enjoy.