With a mixer, beat heavy whipping cream until thick. Fold a cup of it into a bowl with the sweetened condensed milk, add chocolate syrup and optional vanilla and mix until smooth.
14 ounce can sweetened condensed milk, 2 cups cold heavy cream, 1 cup chocolate syrup, 1 teaspoon vanilla
Next, gently fold the rest of the whipped cream into the chocolate mixture until smooth and creamy.
Next, pour half of the ice cream into a freezer safe container. Add half of the mix-in of your choice (if any), and use a spoon or a knife to swirl the mix-in together with the ice cream (optional).
2 king size chocolate candy bars, broken into bits, 6 crushed Oreos, 6 ounces chocolate chips, ¼ cup chopped nuts
Top with remaining ice cream, and then top that with the remaining mix-in.
Freeze for 5-6 hours or overnight. When it is time to serve, remove from freezer and let sit on the counter to soften for 5-10 minutes prior to serving.