Heat oven to 350° F. Butter a 9-inch square baking dish. Slice the plums into a bowl. Sprinkle with ¼ cup of the sugar and toss; set aside.
In a small saucepan, over low heat, warm the milk and vanilla. Add the butter, zest, and salt. Bring to a simmer. Whisking constantly, slowly stir in the polenta. Cook, still whisking, for 1 minute. Transfer to a large bowl. Add the honey, egg yolks, and 3 tablespoons of the remaining sugar and whisk until combined. Let cool until lukewarm.
In a large bowl, with an electric mixer on medium, beat the egg whites and the remaining sugar until soft peaks form. With a spatula, fold the egg white mixture into the polenta mixture.
Pour the batter into the prepared dish. Bake until golden, about 45 minutes. Let cool for 20 minutes. Slice and top with the plums and whipped cream.