STEP 1: WHIP THE BUTTER TO PALE CLOUD
Place the softened butter into the mixer bowl. Whip on medium-high speed for 5 to 7 minutes, stopping twice to scrape down the bottom and sides of the bowl with a silicone spatula. Continue whipping until the butter is noticeably voluminous, aerated, and has lightened from ivory to an almost pure porcelain white.

STEP 2: STREAM THE CONDENSED CREAM IN ADDITIONS
With the mixer running on medium speed, drizzle the room-temperature sweetened condensed cream into the whipped butter in 4 to 5 separate additions (roughly 2 to 3 tablespoons at a time). Whip for 15 to 20 seconds after each addition to fully incorporate before adding the next.

STEP 3: WHIP TO EMULSION
Once the last pour of condensed milk is added, stop and scrape down the bowl thoroughly. Add the pure vanilla extract and pinch of fine sea salt. Whip on medium-high speed for another 60 to 90 seconds until thick, glossy, and cohesive.

STEP 4: SMOOTH THE AIR BUBBLES
Switch the mixer attachment from the whisk to the flat paddle attachment. Beat on the lowest "stir" speed for 2 to 3 minutes. This gently compresses and knocks out excess microscopic air pockets, transforming the whipped texture into a silky, glass-smooth frosting that glides effortlessly across cakes.

STEP 5: ASSEMBLE & DECORATE
Use immediately at room temperature to crumb-coat, fill, or pipe crisp details on cakes and cupcakes.

THE GOLDEN TEMPERATURE RULE:
If the butter and condensed milk are not at the same temperature, the emulsion will break. If the mixture looks curdled or broken, the butter was likely too cold—gently warm the base of the mixer bowl with a warm towel or hair dryer for 10 seconds, then re-whip until smooth. If it looks soupy or greasy, the butter was too warm—chill the bowl for 10 minutes in the fridge, then re-whip.

COLOR BALANCE:
Because of natural dairy butterfat, the frosting will have a warm ivory cast. For a clean white finish, dip the tip of a toothpick into violet gel food coloring and swirl it into the frosting during Step 4 to neutralize yellow tones.

STORAGE:
Store in an airtight container in the refrigerator for up to 7 days, or freeze for up to 1 month. Always bring back to room temperature before re-whipping gently with the paddle attachment to restore its silky texture

FLAVORING:
Use extracts, melted chocolate, caramel, dulce de leche. Some people like the taste of this frosting with no flavorings added because the sweetened condensed cream does have a lot of flavor.
Chocolate: Add 60 g melted and cooled dark chocolate
Mocha or Espresso: Add 2 tablespoons espresso powder
Caramel: Drizzle in soft, pourable caramel
Fruit: Add 2 tbsp fruit puree

SERVING
It is best to use Russian Buttercream within just few hours of making it. (Since it’s so easy to make, there’s no need to whip up large amounts!) You can store it though if you like.

If you add the frosting to a cake or cupcake that must be displayed out for an event, don’t let it set out too long or it will lose consistency. It needs to be kept cool.

This frosting is finicky about temperature. Do not let it set out for much time at all in a warm room.

This is definitely not a frosting for summer picnics.


TIPS

CONDENSED MILK
Different brands of sweetened condensed milk are different colors. I've found that Nestle or Carnation are much lighter color than Eagle Brand. Keep this in mind if you prefer a lighter looking frosting.

SOFTNESS VS FIRMNESS
The more sweetened condensed milk you add, the softer the frosting will be.
The more butter, the firmer it will be.

SWEETNESS
While you can add powdered sugar if you want it sweeter, be careful and taste it first as sweetened condensed cream is already very sweet.

MIXING
Mix the sweetened condensed milk into the butter little by little in small portions - about a tablespoon at a time.

SERVING CHILLED BUTTERCREAM
If you chill it to apply to desserts later you need to whip it a little to get it loosened up.

REWHIPPING BUTTERCREAM
It cannot be too warm at room temp.

TROUBLESHOOTING:

SEPARATION
Your frosting might separate if:
The ingredients are not the same temperature when you mix them.
You try to mix in the sweetened condensed milk too quickly.
The butter or the mixture is too warm at room temp. Even room temp in the bowl if it is too warm and you try to whip it or stir it, it will separate. It must be cool (not cold) to re-whip or re-stir it. 

GREASY, CURDLED, BROKEN
If your buttercream looks greasy, curdled, or broken at any time during this process, it's likely the butter was too cold. Just keep mixing; eventually, the butter will warm as it mixes.

VARIATIONS:

MASCARPONE RUSSIAN BUTTERCREAM
Whip 8 oz of mascarpone cream cheese in a bowl until it is light and fluffy. Mix the whipped mascarpone with the whipped butter a little at a time. Continue the recipe above as written.