Heat oven to 350°. Coat two 8-inch round cake pans with nonstick cooking spray. Line pans with wax or parchment paper; coat paper with spray.
Combine flour, sugar, baking powder, salt, 1/2 cup of the milk and the shortening in a large bowl. Beat at low speed until blended, then beat on high speed 2 minutes.
Combine remaining 1/2 cup milk, the lemon juice, lemon zest and eggs in a small bowl. Gradually add to batter and beat on high speed 2 minutes longer, scraping down sides of bowl occasionally. Divide batter between pans.
Bake at 350° for 33 to 35 minutes or until a toothpick inserted in center tests clean. Cool cakes in pans on racks 15 minutes. Remove cakes from pans to racks and remove paper. Let cool completely.
While cake cools, with an electric mixer, beat butter until smooth. Add confectioners' sugar, half-and-half and vanilla and beat until smooth. Tint frosting pink with food coloring. Set aside.
Place one cake layer on a serving plate. Spread top with 1 cup of the frosting. Gently press second cake layer on top. Spread remaining frosting on side and top of cake. Press M&M's into cake to resemble flowers, alternating colors of centers and petals.