Heat oven to 350°F. Grease two 9 x2-inch round baking pans.
Prepare cake mix according to package directions. Pour batter into prepared pans, dividing equally.
Bake in 350°F oven for 30 minutes or until wooden pick tests clean. Cool cakes in pans on wire rack 5 minutes. Remove cakes to rack to cool completely.
Tint 2 cups frosting yellow. Tint remaining 1 cup frosting green.
Place 1 cake layer, flat side up, on plate. Slide waxed paper strips under bottom edges of cake. Spread 3/4 cup yellow frosting over top. Top with second layer, flat side down. Trim top of cake level. Frost sides of cake with 3/4 cup yellow frosting. Spoon remaining yellow frosting into pastry bag fitted with medium-size round tip. Frost top of cake with 3/4 cup green frosting. Pipe yellow dots around top edge of cake.
Arrange cookies on top of cake. Spoon remaining 1/4 cup green frosting into pastry bag fitted with grass tip. Pipe grass around bottom of cookies. For fence, attach pretzels with dabs of frosting. Arrange on cake top. Pipe grass around fences. Refrigerate until serving time.
1. Heat oven to 350°F. Coat baking sheets with nonstick cooking spray.
2. Roll 1/2 of 18-ounce package refrigerated sugar-cookie dough out with floured rolling pin on well-floured surface to 1/4-inch thickness. Using 3-3/4-inch sheep or other animal cookie cutter, cut out cookies. Place 1 inch apart on prepared baking sheets. Repeat with remaining dough.
3. Bake in 350°F oven 8 to 10 minutes or until lightly golden around edges. Cool cookies on baking sheet on rack 5 minutes. Remove cookies to wire rack to cool completely.
4. To glaze cookies: Reserve 1/2 cup icing. Thin remaining icing with water, adding drop at a time, until consistency of sour cream. Tint if desired. Using small fine-tipped paint-brush, start at one edge of cookie and brush thinned icing across to opposite edge. Add a little more water if icing does not flow smoothly. Let set several hours or overnight. Tint reserved unthinned icing. Frost and decorate cookies as desired.
Use packaged meringue powder to prepare 1 recipe Royal Icing, following directions on package label. Or mix 2 tablespoons powdered egg whites with 6 tablespoons water. Beat in 1 box (16 ounces) confectioners sugar (about 4 cups if sifted); beat about 6 minutes or until good spreading consistency.