The Plum Marmellata:

Bring sugar, lemon juice, salt, and two of the peeled & cut plums to a simmer in a medium saucepan over medium heat and cook, stirring occasionally, until the mixture is thickened and some of the fruit is starting to fall apart, 10–15 minutes.

Mix remaining plums into marmellata and cook, stirring occasionally, until softened but still holding their shape, 5 to 10 minutes. Remove from heat, cover marmellata and chill.

Marmellata can be made 3 days ahead and chilled.

The Polenta Base:

Bring 2 cups water to a boil in a medium saucepan. Whisking constantly, gradually add cornmeal or polenta; whisk until smooth.

Reduce heat to low and cook, whisking and breaking up any lumps, until thickened, about 3 minutes. Whisk in granulated sugar, brown sugar, and salt (mixture will loosen) and cook, whisking often, until mixture thickens again and polenta is tender, 20–25 minutes. If using cornmeal, this will take a little less time.

Remove from heat and whisk in butter. Whisk egg yolk and cream in a small bowl to combine, then vigorously whisk into polenta. Transfer mixture to a large bowl and cover with plastic wrap, pressing directly onto surface. Let cool.

Polenta can be made 2 days ahead. Chill. Bring to room temperature before serving.

Marmellata And Assembly:

Stir polenta to loosen and smooth out any lumps. Spoon into bowls and top each serving with some marmellata