Heat the oven to 375ᵒF. Butter and flour a 9-inch spring form pan. Set aside. In a small mixing bowl, combine the poppy seeds and the hot milk. Set aside until needed.

In a medium-sized mixing bowl, whisk together the flour, baking powder, baking soda and salt.

In a large bowl, using an electric mixer, beat the egg whites until firm but moist peaks form. Transfer the egg whites to a small mixing bowl. Using the same bowl as for the egg whites 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 buttermilk 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 before slicing.