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.
1/3 cup sweetened condensed milk
Add cocoa powder and salt. Whip again until incorporated, 30 seconds.
1 tablespoon cocoa powder, ¼ teaspoon coarse kosher salt
Notes
Note 1: You can use either unsweetened Dutch-processed or natural cocoa powder. Dutch-processed cocoa powder will produce richer and darker-colored frosting.Storing Tips: - Store the chocolate 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.