The cupcakes are sweet not over-powering but sweet. The addition of whipping cream to the buttercream makes it light. To control the sweetness my suggestion to you would be to start with 1/3 cup of condensed milk. Beat and taste the buttercream and then add more if you like. Unlike the whipped cream frosting, buttercream is more stable so can be adjusted a couple of times. Adjust the buttercream to your liking and then add Whipping Cream.