Filling

First, make sure that the butter is at room temperature before continuing with the recipe. To make it lighter in color and fluffy, whip it with an electric mixer after it has reached room temperature. This will take around 3 to 4 minutes of your time to complete.
In this step, add the caramelized condensed milk in several stages, gradually increasing the amount each time. Combine the ingredients well to get a smooth and texture-friendly consistency.
It's time to add the cocoa powder.
Important tip: Once the liquid is ready, place it into a container with a tight-fitting lid. Allow for 2 hours of resting time in the refrigerator, and then remix it when you take it from the fridge.
It is presently complete and ready for filling!
Note: keep in mind that this filling should be put on the cake when it is still cold; unfortunately, it will become loose and difficult to deal with if it warms up because butter softens when exposed to air at average room temperature.