Mix the wet ingredients first: condensed milk, vanilla extract, and butter together. Gradually add cocoa powder to the mixture in a bowl. 
Using a handheld mixer, whip the heavy cream until it thickens. Gradually fold in the cocoa mixture with a spatula. 
Once everything is well incorporated, place the mixture into the freezer for about 4-6 hours or overnight. Enjoy with your favorite garnishes or in a cone!