In a mixing bowl with paddle attachment, beat butter on medium high speed until fluffy, about 3 minutes.
1 stick unsalted butter
Reduce the speed to low and slowly pour in the sweetened condensed milk. Then increase the speed to medium high and continue to beat until smooth and fluffy, about 2 minutes. (Alternatively, if using a hand mixer, add the sweetened condensed milk in 3 batches.)
1/3 cup sweetened condensed milk
Stir in vanilla extract and salt. Whip again until incorporated, 30 seconds.
1 teaspoon pure vanilla extract, ¼ teaspoon coarse kosher salt
Notes
Tips: - Make sure both butter and sweetened condensed milk are at the same temperature. - Butter should be at room temperature, which is around 65°F. Too cold or too soft butter will cause curdled frosting. - The basic ratio of butter to sweetened condensed milk is 1:1. - You can easily double or even quadruple this recipe. - To prevent the frosting from separating, add sweetened condensed milk slowly into the whipped buttercream to allow the butter to absorb the sweetened condensed milk and emulsify properly. Slowly pour the sweetened condensed milk while mixer is running on low. Or if you’re using a hand mixer, add the sweetened condensed milk in 3 batches.How to fix separated/ curdled Russian buttercream: If the frosting is curdled/separated, identify whether your butter was cold or too soft. - If butter was too cold, keep beating it until butter is sufficiently softened. It’ll smooth out as you whip it more. - If the butter was too soft, put the frosting in the fridge to firm up for 5-10 minutes and whip again.Storing Tips: - Store the vanilla Russian buttercream in an airtight container in the refrigerator for up to one week. - Let the frosting come to room temperature. Then, whip until smooth before using.