Step 1: Make the Brownies : Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper. In a saucepan, melt the butter over low heat. Remove from heat, and whisk in sugar and cocoa powder until smooth. Add eggs one at a time, mixing well after each addition. Stir in flour, salt, and vanilla until combined. Pour into the pan and bake for 15–18 minutes, or until just set (slightly fudgy). Cool completely, then cut into small chunks (~½-inch pieces). Set aside.

Step 2: Prepare the Ice Cream Custard Base : In a saucepan, combine milk, 1 cup of heavy cream, sugar, cocoa powder, and salt over medium heat. Whisk constantly until sugar and cocoa dissolve (about 3–4 minutes). In a separate bowl, whisk the egg yolks until slightly thickened. Gradually pour a small amount of the warm milk mixture into the yolks while whisking (to temper the eggs). Pour the tempered egg mixture back into the saucepan and cook over low heat, stirring constantly, until the custard thickens enough to coat the back of a spoon (around 170–175°F / 77–80°C). Remove from heat and add the chopped dark chocolate. Stir until melted and smooth. Stir in the remaining 1 cup of heavy cream and vanilla extract. Pour the mixture through a fine mesh sieve into a clean bowl to remove any bits.

Step 3: Chill the Custard : Cover the bowl with plastic wrap (touching the surface to prevent a skin). Refrigerate for at least 4 hours or overnight until completely cold.

Step 4: Churn the Ice Cream : Pour the chilled custard into your ice cream maker and churn according to manufacturer’s instructions (usually 20–25 minutes). When the ice cream reaches a soft-serve consistency, gently fold in the brownie chunks using a spatula.

Step 5: Freeze Until Firm : Transfer the ice cream to a freezer-safe container. Press parchment or wax paper directly onto the surface. Freeze for at least 4 hours, or until firm and scoopable.