1. Preheat oven to 350 degrees. Butter two 8-by-2-inch round cake pans, and line with parchment paper. Butter parchment, and dust bottoms and sides with flour, tapping out excess; set aside.

2. Into a medium bowl, sift together flour, baking powder, and salt twice; set aside. In a mixing bowl, using the paddle attachment, cream the butter on low speed until soft. Increase speed to medium; beat until pale and fluffy.

3. Gradually add 1 1/2 cups sugar; beat until fluffy, about 3 minutes. Beat in vanilla, poppy seeds, and lemon zest. With mixer on low speed, add the flour mixture in two batches, alternating with the milk; beat until thoroughly combined after each addition. Scrape sides of the bowl; beat 10 seconds more. Set aside.

4. In another mixing bowl, whisk the egg whites and cream of tartar on low speed until foamy. Increase speed to medium-high; gradually add remaining 1/4 cup sugar until peaks are just stiff. Gently fold in whites; do not overmix.

5. Divide batter between prepared pans; bake until golden and a cake tester inserted in the centers comes out clean, about 40 minutes. Transfer to a wire rack to cool completely. To remove cakes, invert onto plates; peel off parchment paper, and reinvert so the top side is up.

6. Make filling: In a small bowl, whip heavy cream to soft peaks. Fold in lemon curd; cover with plastic, and refrigerate until chilled and firm, at least 2 hours.

7. Assemble cake: Using a serrated knife, carefully trim tops of both cakes to make level, if desired. Slice each in half horizontally to make a total of 4 layers. Brush away crumbs from tops of layers.

8. Place one of the bottom layers on a serving platter; spread with 1 cup lemon filling. Top with another cake layer; spread with 1 cup filling. Repeat with another cake layer and filling; top with remaining cake layer. Cover entire cake with plastic wrap, and carefully transfer to the refrigerator, steadying it with your hands to keep the layers from sliding. Chill until filling is firm, at least 2 hours.

9. When ready to serve, frost cake and garnish with poppy seeds.