Make the custard base. In a medium bowl, whisk the egg yolks and the sugar until it turns a light yellow color. In a medium saucepan, combine the heavy cream and the milk. Heat the milk over medium-low heat, until it begins to simmer. Remove from heat.
Temper your egg mixture. Slowly add milk and heavy cream to eggs and sugar, stirring constantly, so the eggs don’t begin to cook and curdle on you. Add the mixture back to the saucepan.
Heat custard. Return to heat until temperature reaches 160-165° F. Stirring constantly so the eggs in the custard don’t cook on the edges of the pan. Visually, you’ll know the custard is done once it starts to thicken. (I use a thermometer gun to temp at the surface and have found that taking it off when it reads 160° F ensures nothing curdles. I also try to move any accumulated foam out of my way with a spatula as the custard below seems to be hotter.) For me, using my largest (gas) burner, this takes less than 6 minutes. Remove from heat.
Let your custard base mature. Pour custard through a fine-mesh sieve, and store either in a bowl covered with plastic wrap or in an airtight container in the fridge for 4 hours or overnight.
Churn the custard. Once the custard base is chilled, remove from fridge and add pure vanilla extract. Pour it into your ice cream maker and proceed according to your ice cream maker’s instructions. (With my Cuisinart Compressor Ice Cream Maker, this takes 30-35 minutes.)
Pour into an ice cream container, loaf pan, or tupperware container. Make sure fudge is room temperature. We’re going Mama Bear here - not too cold, not too hot. Just Right. Pour a layer of the vanilla ice cream base into your ice cream container. Then dollop spoonfuls of fudge onto the first base layer. Pour more of the base over the chocolate. Repeat until no more ice cream remains. You’ll use approximately 1 cup of the fudge sauce, with a little leftover to use as ice cream topping. Use a knife or a spoon to swirl everything together, working fast so the ice cream doesn’t melt on you.
Freeze. Put it into the freezer for at least 3 hours or until it reaches your desired consistency. To serve, put the ice cream container in the fridge for 20-30 minutes to soften.