In a bowl of a mixer fitted with a whisk attachment whip room temperature butter, stopping to scrape the bottom of the bowl several times, until it has tripled in volume, and is light and fluffy, about 5 - 7 minutes.
Add 1 teaspoon of vanilla extract or emulsion to the whipped butter. Now add the sweetened condensed milk or dulce de leche in thirds, whipping for about 5-10 seconds after each addition. Don't forget to scrape the bottom of the bowl after each addition.
1. Start out with room temperature butter. Remove your butter from the fridge several hours before you plan on using it. The butter must have some yield when pressed with your finger. If your butter is too soft to begin with, it will not be able to hold onto the condensed milk when it is added, and make the buttercream separate.
2. Whip butter for full 5-7 minutes. Air bubbles whipped into butter will help it hold on to the condensed milk while holding shape. If not enough air bubbles are incorporated, the buttercream will become runny when you add the condensed milk.
3. The condensed milk must be added in a slow steady stream as if its hot sugar being added for Italian meringue.
4. Make sure both butter and condensed milk are room temperature, or the same temperature. If you try adding cold condensed milk to whipped room temperature butter, most likely the butter will cool down too much and become hard, preventing it from being smooth, or worse yet – separate.