Combine butter chunks, ½ cup granulated sugar, brown sugar, and cream in 1-quart saucepan. Cook over medium heat, stirring occasionally, until mixture thickens and comes to a full boil (5 to 8 minutes). Stir in vanilla. Set aside to come to room temperature.Make the plum porridge: Chop the plums into small ½-inch pieces, removing the pits as you go. Have a large bowl nearby to collect the plum flesh and any juice that accumulates as you work. Toss the plum pieces with the lemon juice and remaining 4 cups sugar.
Add the apple juice and any accumulated plum juice.
Let the mixture sit about twenty minutes.Choose a large, wide heavy-bottomed pot.
Add the plum mixture and bring it to a low, slow boil over medium-low heat heat. This could take a half-hour or more. Do not rush it. Stir the mixture frequently scraping the bottom of the pot to make sure the fruit doesn’t stick or burn. Once the low boil is achieved adjust the heat and allow it to simmer an additional 20-25 minutes, skimming off any foam that develops on top. The texture is important, you don’t want to overcook or over stir the fruit to the point that it breaks down too much.Create a slurry with the potato starch. Turn the heat off and slowly drizzle in the starch, stirring the whole time. Cover the pot and let it sit until it comes to room temperature. The porridge is served at room temperature with a drizzle of vanilla sauce on each portion.
