Mix and bake the cake mixes according to the box instructions.
In a microwave safe bowl add the chopped chocolate and microwave for 30 seconds. Microwave in 10-second intervals, stirring between each, until chocolate is completely melted. Set aside to cool slightly.
In a large bowl add the softened butter, salt and vanilla extract. Mix until smooth.
Add the powdered sugar and mix until combined. Frosting will be thick.
Add the melted chocolate and mix. Add the heavy cream and mix until completely combined and smooth. Put frosting into a piping bag and set aside.
Take the cooled cake and chop into cubes. Add the chopped cake to the bottom of the jar. Top with frosting and assorted sprinkles of your choice. Continue layering until the jar is full.
Top the jars with holiday fabric and tie with string. Perfect holiday gift!