In a saucepan over medium heat, mix thawed black raspberries and lemon juice. Cook for 5-7 minutes until it thickens. Strain to remove seeds, so you have about ½ to ⅔ cup of puree. Let it cool.

In another saucepan, heat whole milk with half the sugar until it steams. In a bowl, whisk the egg yolks with the remaining sugar until pale. Slowly add the hot milk while whisking.

Pour it back into the saucepan and cook on low until it thickens (170°F-175°F). Stir in vanilla and raspberry puree, then strain for a smooth mixture. Put the custard in a bowl, cover it with plastic wrap, and refrigerate for at least 4 hours or overnight.

In a saucepan on low heat, melt cocoa powder, corn syrup, butter, chocolate chips, cream, and salt until smooth. Let it cool to room temperature. Churn the chilled custard in the ice cream maker according to the machine's instructions (about 20-25 minutes) until it thickens.

In a freezer-safe container, layer the ice cream and fudge swirl. Use a knife to create ribbons without fully combining them. Cover the surface with plastic wrap, seal the container, and freeze for at least 4-6 hours. Let it sit at room temperature for 3-5 minutes before serving to make scooping easier.