Add 1 (14 oz) can sweetened condensed milk and 1 cup semi-sweet chocolate chips to a medium saucepan. Heat over medium heat, stirring constantly, until chocolate chips are melted. This will take 3-5 minutes.: The moment the sweetened condensed milk meets the semi sweet chocolate chips you will notice a glossy sheen begin to form as the chocolate warms. Start over medium heat and allow the mixture to come together slowly, the surface will shift from dull to satiny, and small pockets of melted chocolate will appear. Stirring frequently spreads heat evenly and prevents hot spots, which would otherwise scorch and create grainy texture. One common mistake is turning the heat up too high, which causes the chocolate to seize and separate. If you hear little popping or see oil separating, lower the heat and stir vigorously to coax it back together. Keep the pan centered on the burner so the base heats uniformly.
Remove from heat and stir in 2 tablespoons unsalted butter, 1/8 teaspoon sea salt, 1 teaspoon vanilla extract, and 3 tablespoons heavy cream. If needed, add additional heavy cream to thin.: You will know the chips are melted when the mixture flows smoothly and you no longer see intact chip shapes. The stirring rhythm should be gentle but steady, almost meditative, scraping the bottom and sides so nothing clings and burns. The aroma will intensify, smelling like warm cocoa with sweet dairy notes. If the sauce seems sluggish or grainy, finish melting off heat and continue stirring, the residual warmth will help it smooth out. Avoid walking away, because even a brief gap can let localized scorching begin at the bottom of the pan.
Serve warm or allow to cool for storage.: Once off the burner, the butter should be added to enrich texture, and you will see the sauce become silkier as the butter melts into it. Stirring at this stage allows the vanilla and sea salt to bloom into the warm chocolate, releasing aroma and deepening taste. Adding the heavy cream makes the sauce glossy and pourable, and you should notice the surface take on a mirror like shine. A typical pitfall here is adding cold cream too quickly, which can momentarily stiffen the sauce; let the cream sit at room temperature briefly if possible, or add it slowly while stirring to maintain smoothness.
If needed, add additional heavy cream to thin: Texture adjustments happen at the end, and this is when you control how the sauce will behave once plated. For a thick spoonable fudge let it cool slightly, for a drizzle add small amounts of warm heavy cream until the consistency is right. The sauce should coat the back of a spoon without running off immediately if you want a richer finish, or it should flow in a steady ribbon for drizzling. A mistake is adding too much cream at once; do it gradually and test, because it is easier to thin than to thicken back without additional chocolate.
Serve warm or allow to cool for storage: Serving warm gives the richest aroma and that comforting, molten texture that pairs beautifully with cold ice cream . As it cools the sauce will thicken and become more like a ganache, which is excellent for filling or spreading. For storage, let it cool to near room temperature before transferring to an airtight container to refrigerate. A common error is sealing very hot sauce in a container, which can trap steam and alter texture; cool it a bit first, then rewarm gently when ready to use.
