Hi all! So I’m a novice baker and I’m still learning so plz don’t judge me too harshly lol anything I did I found on the www. Anyway I made American buttercream using wickedgoodkitchens recipe which was basically to mix PS and boiling water until the ps is basically like dissolved and then add butter and shortening (and salt and vanilla). Not sure what I did wrong (although it’s getting hotter here in Cincinnati so I’m not sure if it’s the kitchen temp) but the frosting is just too slippery. If I pick it on my spatula it’s slides off. I’ve added more sugar and some cornstarch mixed in milk and without milk (lol

I'm thinking you added too much liquid to the mixture. Depending on the consistency you could add PS until it firms up, but without seeing it I'm not 100% sure that would work. 

I’ve never seen an American buttercream where you dissolve the icing sugar. 

It’s usually made by beating the butter then adding double the amount of icing sugar (by weight). Add cream or milk to thin it and then vanilla and salt to taste. 

I agree w/louglou - have never heard of an ABC where you dissolve the icing sugar.   I bet you would be much happier using this recipe:  https://www.cakecentral.com/recipe/22469/2-icing

I agree with all of the above, American buttercream does not use boiling water to dissolve the powdered sugar.  Cream the shorting, whether it be butter, Crisco, or a combination of the two, until it is light and fluffy, then start gradually adding powdered sugar.  When it gets too stiff, add cool water, one tablespoon at a time until all the sugar has been added. Before the last of the sugar is incorporated, add flavorings.

American Meringue Buttercream

Place Cream and Egg Whites in 4.5 qt.Mixer and whip on High with whisk attachment until light and frothy, 30 seconds to 1 minute.

Add in Meringue Powder, Salt and Powdered Sugar and whip on high for 1 min. stop & scrape sides. Beat 4 minutes more.

Continue mixing on low and add butter in 1 stick at a time, then add in Vanilla Extract and Butter Flavor.

Whip entire mixture on high for 6 minutes.

Switch to Paddle Attachment and mix on low for 3-5 minutes or until most of the air bubbles have been released.

Hay guys did anyone notice the date this was posted?:)   Of course it never hurts to expound on a topic since someone out there might be wanting to know & too shy to ask

So I think after I posted this, I found cupcake jemmas recipe on YouTube and that’s the first time I made ABC perfectly