Preheat oven to 350°F (180°C). Spray 3 (8-inch) round cake pans with baking spray with flour. Line bottom of pans with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, whisk together flour, sugar, poppy seeds, lemon zest, baking powder, salt, and baking soda by hand. Add butter; beat at medium-low speed until butter is incorporated and mixture resembles coarse crumbs, about 2 minutes.

In a medium bowl, whisk together milk, eggs, and vanilla extract. With mixer on low speed, add one-third of milk mixture (about ⅔ cup or 130 grams) to flour mixture. Increase mixer speed to medium, and beat until light and fluffy, 1 to 2 minutes. Gradually add remaining milk mixture, beating until well combined and stopping to scrape sides of bowl. Divide batter among prepared pans (about 2 cups or 470 grams each).

Bake until a wooden pick inserted in center comes out clean, about 25 minutes. Let cool in pans on wire racks for 10 minutes. Remove from pans, and let cool completely on wire racks.

Spoon 1 cup (220 grams) Vanilla Buttercream into a pastry bag; cut a ¼-inch opening in tip.

Level cooled cake layers, if desired. Place 1 cake layer on a serving plate. Spread ¼ cup (55 grams) Vanilla Buttercream in a very thin layer on top; pipe a small border of buttercream along edge of cake layer. Spread half of lemon curd within border. Top with second cake layer. Spread ¼ cup (55 grams) buttercream in a very thin layer on top; pipe a small border of buttercream along edge of cake layer. Spread remaining lemon curd within border. Top with remaining cake layer. Spread remaining buttercream on top and sides of cake as desired. Garnish with poppy seeds and lemon twists, if desired. Refrigerate for 30 minutes before serving. Refrigerate in an airtight container for up to 3 days.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter and salt at medium speed until smooth and creamy. With mixer on low speed, gradually add confectioners’ sugar, beating until combined and stopping to scrape bottom and sides of bowl. Beat in cream and vanilla. Increase mixer speed to high, and beat until pale and fluffy, 1 to 2 minutes. Use immediately.