Beat the cake mix and soda for 2 minutes in a bowl. Add the food coloring if using.
Put the pan in the oven and bake for 25-30 minutes or until a toothpick comes out clean.
Let the cake cool before adding the frosting.
To make the frosting, put 1 cup of the soda into a saucepan and heat until boiling. (over medium-high heat) Once it is boiling, reduce to medium-low and let it simmer to 1/4 of what it was. (this will leave about 2 ounces of the soda)
Let it cool down entirely.
Grab a bowl and beat the butter and powdered icing sugar for 2 minutes. Then add the soda.
Beat to mix, then add the milk as needed until it is spreadable. Add the food dye if you want.
Use an offset spatula to add the frosting to the cooled cake.
If you want to add texture to the frosting, use short strokes to spread it, and then go back over the strokes and swirl it a bit.