I typically use the Extra Special Buttercream recipe I got here on CC, but a client requested something not too sweet. Any suggestions on a crusting buttercream that is not as sweet as this one? Thanks!

What's 'too sweet'? It's TOTALLY different to each person.
There are a few things one can do to help cut the sweetness in b'cream recipes.
1) reduce the sugar by 1/2 cup
2) replace 1/2 cup sugar with 1/4 cup cornstarch
3) add up to 1/2 teaspoon salt
4) add more flavoring. Most recipes don't have enough flavoring to begin with. Use up to 2 Tablespoons. Make a mixture of 3 different flavors. Example: 1 teaspoon lemon flavor or juice with 2 teaspoons orange OR almond flavoring and 3 teaspoons vanilla. Don't be afraid to try new and different combinations until you find what is pleasing to you and your customers.

adding some lemon flavoring will cut down some of the sweetness. Or you can try IMBC or SMBC - they're both delicious and not too sweet.

My suggestion SMBC . Tried it for the first time this weekend . Not as sweet as regular buttercream but amazing !

Crusting buttercreams by their very nature are VERY sweet due to the high sugar to fat ratio. To make it less sweet, you have to cut that ratio (then it won't be crusting) or use a SMBC.