Melt the whole dark chocolate bar over a double boiler. (Make your own double boiler by setting a heat-resistant, metal dish over a pot of simmer water.) Slowly whisk in half-and-half and heavy cream until well combined. Bring mixture to a simmer, stirring often as it cooks, then remove from heat and set aside.
In a small bowl, mix cocoa powder and sugar. Set aside. Meanwhile, whisk egg yolks in a large mixing bowl until their color begins to brighten, approximately two minutes. Slowly add the cocoa-sugar mixture and beat until well combined.
Mix in about one-third of the warm chocolate mixture to the eggs and sugar. Once mixed together, scrape the eggy liquid back into the saucepan with the remaining warm ingredients. Over low heat, cook mixture to thicken, stirring frequently. It should reach about 170 to 175 degrees, or be thick enough to coat the back of a spoon. Remove from heat and let cool to room temperature, about 30-45 minutes. Once cool, stir in the vanilla, place in a glass or plastic lidded container and refrigerate for at least 4 hours or overnight.
When ready, churn cooled mixture in your ice cream maker according to the appliance instructions, about 25-30 minutes. Fold in the peanut butter cups and dark chocolate. Serve immediately or freeze for 4 hours to allow ice cream to firm.