The recipe I use is 400g sugar, 8 egg whites (from a carton, because I hate wasting yolks), 1/2 cup water, 450g butter and a tsp of vanilla.
Every time I've made it, the buttercream firms up very quickly, and I have to keep microwaving it to make it useable (even if it's a fresh batch). Even after leaving the cake out at room temp for an hour, the buttercream is very solid, almost like eating regular butter. Is this typical of IMBC or am I doing something wrong? I've never tried IMBC made by anyone else, so I have no idea if this is what it's supposed to taste like. Any advice would be appreciated :)
you made a meringue with the egg whites -- yes?
I use 400 g sugar (14 oz), 10 whites (1.5 cups) and 6 cubes of butter (680 grams, 24 ounces). Whisk the whites with the sugar over a water bath until 150 - 160º F. Whip the whites in a mixer on med - hi speed until they are a fluffy stiff meringue that is completely cooled. May take a while depending on room temp. Blend in softened butter on low until smooth (I swap from the whisk attachment to the regular mixer attachment) then mix in flavorings or additives. I have used carton whites, but don't like the flavor quite as much, although it works fine, especially for chocolate or stronger flavors. Many recipes use even more butter than this one - I think mine is on the lower end of butter amounts. Maybe yours is a bit light on the butter? I also have never seen a recipe with added water. Not sure how that would alter the chemistry of it, but something is not working for you. Consider changing up your recipe a bit.
In my hands this one always works, always yummy, NEVER firm at room temperature (assuming your room temperature is not freezing!) HTH