To make the flower: Line a rimmed baking sheet with parchment paper, dust the work surface with corn starch. Roll the gum paste to 1/8-inch thickness and cut out the desired number of flowers using the daisy shaped cookie cutters. Place the flowers on the baking sheet. Let stand, uncovered and at room temperature overnight.
Cake: Preheat oven at 350°F. Line three 8-inch round cake pans with parchment paper. Grease the paper lightly, whisk the flour, baking powder and ½ tbsp salt in a bowl.
Beat 2 sticks of butter with an electric mixer on medium speed for 2-3 minutes until it is light and fluffy. Gradually beat in granulated sugar, beat for 2-4 minutes until light and fluffy. Add egg yolks, one at a time until mixed well. Beat in 1 tsp vanilla extract. Reduce the speed of the mixer to low and beat in the flour mixture and sour cream, begin with some flour mixture, then sour cream and then end with flour mixture. Beat in milk and 1 tbsp grated lemon zest, plus 1/4 c. lemon juice
Beat egg whites in a clean bowl on high speed until soft peaks form. Stir 1/3 egg whites into batter. Gently fold in remaining egg whites. Divide the batter among prepared pans. Bake until cake is golden brown and edges pull away from sides. Cool in pans on wire racks for 10 minutes and then invert onto racks to cool them completely
Make frosting: beat the rest of the butter on medium speed until creamy. Reduce the mixer speed to low and gradually beat in normal sugar. Beat in vanilla, salt, cream and lemon zest and juice. Increase speed to medium and beat until fluffy
Place one layer of cake on a cake stand or plate. Top with ¾ cup frosting and spread to edges. Repeat one more time. Top with third layer. Frost top and sides with remaining frosting and chill for 30 minutes.
Cut lemon jellies in half horizontally. Cut into small rounds using large opening of a piping tip. Attach to centers of flowers, sticky sides down. Cut fruit leather into 12 (1/4-inch-wide) strips of various lengths for stems. Cut out some small leaves with scissors. Arrange flowers on cake (placing some toward the top of the cake so they overlap the edge), pressing into icing to adhere. Add a stem (trimming length as necessary) and leaves for each flower, pressing gently to help them adhere. Scatter fruit-leather grass around cake, if desired.