Meanwhile, make the dulce de leche frosting: In a large bowl of stand mixer, add the softened unsalted butter, powdered sugar, dulce de leche, vanilla extract, and heavy cream. Using the paddle or whisk attachment, beat on low for a few seconds until the sugar is incorporated (this will prevent the sugar from flying all over). Then, increase speed to medium-high to high and beat for about 2-3 minutes, until fluffy. Cover with plastic wrap, making sure the plastic wrap touches the frosting, and refrigerate for 30 minutes. It makes the frosting firmer and makes it easier to pipe.