In a large bowl, cream butter and sugar. Add eggs, one at a time, beating well after each addition. Combine the sour cream, milk, lemon juice, peel and vanilla; add to creamed mixture. Combine the flour, baking powder, baking soda and salt; add to the creamed mixture and beat just until combined. Stir in poppy seeds (batter will be thick).
Transfer to three greased and floured 9-in. round baking pans. Bake at 350° for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks.
For frosting, in a bowl, beat cream cheese and butter until light and fluffy. Gradually beat in sugar. Beat in enough milk to achieve spreading consistency. Place 1 cup of frosting in a small bowl and tint pale green; set aside.
To assemble, place one cake layer on a serving plate; frost top with white frosting. Repeat. Top with remaining cake layer. Frost top and sides of cake with remaining frosting.
Cut a small hole in the corner of a pastry or plastic bag; insert round tip No. 3. Fill bag with green frosting. Write Happy Easter on top of cake. With multi-opening or grass tip No. 233, pipe green frosting on top of cake to resemble a bird's nest. Pipe green frosting around top edge of cake. Place jelly beans in nest. Store in the refrigerator. Yield: 14-16 servings.