I've written a short-hand version of my peanut butter frosting recipe at the top of the chocolate sour cream cake post, but here are some more detailed instructions. The one I used for the "salty" cake is heavy, sticky, and salty. The one posted here is a little lighter without sacrificing that strong peanut buttery flavor.
Peanut Butter Frosting
Makes enough to frost the layers, sides, and tops of a three-layer, eight-inch round cake.
1/2 cup heavy cream
1 tablespoon sugar, granulated or superfine
1 teaspoon vanilla extract
1 cup smooth peanut butter, the processed kind works better than natural peanut butter because the oil won't separate
1/2 to 1 cup powdered sugar (confectioners' sugar)
3 to 8 tablespoons hot water
Either by hand with a balloon whisk or in a mixing bowl with the balloon whisk attachment, or using electric beaters, beat the cream and granulated sugar on high until the cream begins to hold shape. Do not over beat or you'll make butter. Err on the side of caution and beat a little less than you think you need because you can always whip it once or twice more if necessary. Fold in the vanilla extract and set the whipped cream aside.
In another bowl, either using electric beaters or the paddle attachment in an electric mixer (or a wooden spoon and a lot of muscle power if working by hand), beat together the peanut butter and a half cup of powdered sugar. The mixture will be crumbly. Add a few tablespoons of very hot water, a little at a time, until the peanut butter returns to a smoother texture. It will still be very thick and not spreadable. Add a few more tablespoons of hot water if necessary and give it a taste. If you like very sweet frosting, add more powdered sugar and more hot water until you have the sweetness right. The consistency should still be quite thick.
Turn the mixer to low-medium and add a few scoops of the whipped cream to the peanut butter mixture and blend them together. Add more whipped cream and continue to blend until the frosting lightens in color and becomes softer and more spreadable. You don't need to add all the whipped cream, so taste as you go to get the frosting where you like it. If the frosting has too much whipped cream, it won't hold up well between the cake layers.
Be sure to refrigerate this frosting and any cake that uses it!
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