In a small mixing bowl, combine the poppy seeds and the hot milk. Set aside until needed. Preheat the oven to 375ᵒF and arrange an oven rack in the middle position. Butter and flour a 9-inch spring form cake pan. Set aside.
In a medium-sized mixing bowl, whisk together the flour, baking powder, baking soda and salt.
Place the egg whites in the bowl of an electric stand mixer, fitted with the whisk attachment. Whisk on medium-high until firm but moist peaks form. Transfer the egg whites to a small mixing bowl. Using the same bowl as for the egg whites, but now using the paddle attachment, beat the butter and sugar on medium-high speed until light and fluffy, about 3 minutes. Add the vanilla, then beat in the egg yolks, adding one at a time and beating well after each addition. Scrape down the sides of the bowl with a rubber spatula, as needed.
Drain the milk from the poppy seeds, discarding the milk. Add the sour cream and the drained poppy seeds to the batter. Beat until well combined, then again scrape down the sides of the bowl with the rubber spatula. Add the flour mixture to the batter, in thirds. Again scrape the bowl with the rubber spatula, making sure it’s all well mixed. Fold in about a quarter of the beaten egg whites with the spatula, then fold in the rest, mixing gently until just combined.
Transfer the batter into the prepared cake pan, smoothing the top with the rubber spatula. Bake until golden and firm, with the sides just beginning to pull away from the pan, about 40-50 minutes. Remove from the oven and place on a wire rack. Carefully run a sharp, thin knife along the sides of the cake, just against the pan, then gently remove the rim and allow the cake to cool to room temperature. Slice into 10-12 wedges and serve with roasted plums and whipped cream (See recipes below).
Preheat the oven to 400ᵒF.
Wash and dry the plums, then slice in half, remove the pits, and slice again into 1-inch thick wedges, about 8 per plum. Combine the sliced plums and honey in a large mixing bowl and toss to coat. Add the rosemary sprigs and toss again to coat.
Grease the surface of a large baking sheet with butter, then spread the plums and rosemary evenly over the pan, in a single layer. Dot the tops of the plums with small pieces of the remaining butter.
Bake until the plums are tender, juicy, and just barely turning golden at the edges, about 30 minutes. Remove from the oven, and allow to cool to room temperature before serving. These can be kept refrigerated in an airtight container for up to days, but make sure to re-heat slightly before serving.
For the whipped cream:
Whip the cream to soft peaks (When you lift the whisk out of the cream, a point will form then droop down after a second or two). Add sugar and vanilla and whip to firm peaks (The point formed in the cream will stand straight up and not fall over).