For the ice cream
1 pint heavy whipping cream, chilled
3 tablespoons (15 g) unsweetened Dutch-processed cocoa powder (I buy Rodelle brand (aff link))*
5 ounces bittersweet chocolate, chopped
10 ounces sweetened condensed milk
For the chocolate syrup
3/8 cup (3 fluid ounces) water
3/4 cup (150 g) granulated sugar
1/8 teaspoon kosher salt
1 teaspoon (7 g) light corn syrup (optional—keeps the sugar in the syrup from crystallizing)
6 tablespoons (30 g) unsweetened Dutch-processed cocoa powder*
*If you don’t have Dutch-processed cocoa powder, try adding the very smallest pinch (less than 1/8 teaspoon) of baking soda to natural cocoa powder to neutralize the acid in the natural variety of cocoa powder.
To make the ice cream, in the bowl of a stand mixer fitted with the whisk attachment (or a large bowl with a handheld mixer), place the cream. Beat medium-high speed until slightly stiff peaks form. Add the cocoa powder to the whipped cream, whisking or beat constantly just until it is mostly absorbed into the cream. Place the whipped cream in the refrigerator to chill.
Melt the chopped chocolate in the microwave or in a double boiler (in a heat-safe bowl positioned over a pot of simmering water, making sure the water does not touch the bowl), stirring occasionally until smooth. In a large bowl, mix the melted chocolate with the sweetened condensed milk, and whisk until well-combined and smooth. The mixture will be relatively thick. Allow to cool until cool to the touch.
Scrape about half of the the whipped cream into the bowl of chocolate and condensed milk, and whisk carefully until smooth. Add the remaining whipped cream and, using a silicone spatula, fold it in until very few bright white streaks remain. Pour the mixture into a 2 quart freezer-safe container with a lid, and smooth the top. Cover tightly and place in the freezer until firm (at least 4 hours). Allow to sit at room temperature for about 10 minutes or in the refrigerator for about 15 minutes before serving to make scooping easier.
To make the chocolate syrup, in a medium-size, heavy-bottom saucepan, place the water, sugar, salt, and optional corn syrup, and whisk to combine. Place the saucepan over medium-high heat and bring to a boil. Whisking constantly, add the cocoa powder to the sugar mixture and continue to cook, whisking vigorously, until the cocoa powder is absorbed into the liquid. Lower the heat to medium-low and continue to cook, whisking frequently, until the mixture has begun to thicken and coats the back of a spoon, leaving a clean, visible trail when you run your finger across the syrup on the spoon (about 5 minutes). Remove from the heat and transfer the syrup to a small, heat-safe bowl. Serve warm over the ice cream. Store any remaining syrup in the refrigerator in a sealed bottle or other container.