To make the cake, position a rack in the middle of the oven and preheat the oven to 350 degrees F. Butter and flour 2 9-inch round cake pans. Whisk together the flour, baking powder, salt, and poppy seeds in a bowl; set aside. In the clean, dry bowl of an electric mixer fitted with the whisk attachment, beat the egg whites on medium speed until foamy. Add the cream of tartar and increase the speed to medium-high. Beat just until stiff peaks form when the whisk is lifted. Transfer the egg whites to a separate bowl, clean the mixer bowl and reattach it with the paddle attachment.

Add the butter to the mixer bowl and beat on medium speed until smooth. Gradually add the sugar and beat until incorporated. Mix in the lemon zest. Beat the mixture on medium-high speed until light and fluffy. Add ¼ cup of the milk and beat until just blended. Reduce the speed to low and add the flour mixture alternately with the remaining milk in three batches, beginning and ending with the flour, scraping down the sides of the bowl as necessary. Beat until just blended. Using a rubber spatula, fold one quarter of the egg whites into the batter, being careful not to deflate the mixture. Once incorporated, add in the rest of the whites and gently fold in until well combined.

Pour the batter into the prepared pans and bake until golden and a toothpick inserted in the center comes out clean, about 25-30 minutes. Cool the cakes in the pan on a wire rack for 10 minutes. Run a thin knife around the outside of pans and gently turn the cakes out. Allow to cool completely.

To make the raspberry curd, melt the butter in a large saucepan over medium heat. Add the raspberries, egg yolks, sugar and salt, and cook, mashing the berries. Stir frequently at first and then constantly at the end, until thickened, about 10 minutes. Pour the mixture through a coarse strainer set over a bowl, pressing hard on the solids to extract as much liquid as possible. Cool to room temperature; the curd will continue to thicken as it cools. Stir in lemon juice to taste. Cover and refrigerate until ready to serve.

Layer the rounds with the cooled raspberry curd. Once layered, allow the assembled cake to cool in the refrigerator to help them set. In the meantime, make the frosting. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and zest on medium speed until light and fluffy. Gradually add the sugar and beat until smooth. Add the lemon juice and beat for one minute longer.

Frost the chilled cakes with the lemon frosting. Garnish with fresh raspberries as desired.