Prepare an ice bath by filling a large bowl with ice and water; set a smaller bowl inside it.
In a medium saucepan, whisk together 2 cups (480 ml) heavy cream, 1 cup (240 ml) cashew milk, cocoa powder, and stevia.
Heat over medium heat, whisking occasionally, until the mixture reaches 170°F / 77°C on a candy thermometer.
In a separate medium bowl, lightly whisk the four egg yolks.
While whisking constantly, slowly pour about one cup of the hot cream mixture into the yolks to temper them.
Pour the tempered egg yolk mixture back into the saucepan with the remaining cream.
Return the saucepan to medium-low heat, stirring constantly with a spatula, until the custard thickens and reaches 175°F / 80°C. Immediately remove from heat and stir in the chopped dark chocolate.
Let the mixture stand for 5 minutes to allow the chocolate to melt completely, then whisk until smooth and glossy.
Pour the custard into the prepared ice bath bowl to cool.
Once cool, cover with plastic wrap pressed directly onto the surface and chill in the refrigerator for at least 3 hours, or overnight.
Once thoroughly chilled, whisk the remaining ½ cup cashew milk and the vanilla extract into the custard base. Churn the mixture in an ice cream maker according to the manufacturer's .
Serve immediately for a soft-serve consistency, or transfer to a freezer-safe container and freeze for 2-3 hours for a firmer texture.
