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1
Bring 2 cups water to a boil in a medium saucepan. Whisking constantly, gradually add polenta; whisk until smooth. Reduce heat to low and cook polenta, 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. Remove from heat and whisk in butter.
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2
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.
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3
Bring sugar, lemon juice, salt, and half of plums to a simmer in a medium saucepan over medium heat and cook, stirring occasionally, until mixture is thickened and some of the fruit is starting to fall apart, 10-15 minutes.
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4
Mix half of the remaining plums into marmellata and cook, stirring occasionally, until softened but still holding their shape, 5 minutes. Remove from heat and mix in remaining plums. Cover marmellata and let cool.
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5
Stir polenta to loosen and smooth out any lumps. Spoon into bowls and top each serving with some marmellata and a scoop of vanilla ice cream.
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6
Polenta can be made 2 days ahead. Chill. Bring to room temperature before serving.
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7
Marmellata can be made 3 days ahead. Chill.
