I finally figured out why everyone's buttercream looks so greasy now
After seeing a dozen posts here about split frosting, I realized most people skip the crucial step of letting the butter and cream cheese sit out for a full 90 minutes to properly soften, which is why their mixes turn into a shiny, separated mess.
My grandma's old recipe book says 30 minutes too, but it depends on your kitchen. If it's warm, 30 is fine, but my last place was so cold the butter was still a brick.