Combine oatmeal, brown sugar and butter in large bowl. Pour boiling water over mixture. Let stand until oatmeal is softened and mixture is lukewarm. Stir thoroughly to combine ingredients and to make sure sugar is dissolved.
Meanwhile, heat oven to 350 degrees. Line bottom of two 13 x 9 x 2-inch baking pans with waxed paper. Coat with nonstick cooking spray.
Stir together flour, baking soda, cinnamon, allspice and salt in medium-size bowl.
Add granulated sugar and eggs to oatmeal mixture. Beat at medium speed until smooth, about 1 minute. At low speed, beat in flour mixture just until blended. Scrape batter into prepared pans, dividing evenly.
Bake in 350 degree oven for 30 minutes or until cakes spring back when lightly touched in center with finger. Cool cakes in pans on wire racks for 10 minutes. Remove cakes from pans to wire racks to cool completely. (Cakes can be made ahead up to this point; wrap and refrigerate up to 2 days, or freeze up to 1 month.)
Beat together sugar, butter, vanilla, lemon extract and half-and-half in bowl until smooth and creamy. (Frosting can be refrigerated, covered, for up to 2 weeks. Bring to room temperature before using.)
Cut cakes in half lengthwise down center. Stack 3 halves, spreading frosting in between layers. (Save fourth layer for another use.) Trim all sides of cake, angling down slightly, as you slice, so that cake is wider on top than bottom.
Frost side and top of cake. Spread frosting on back sides of cookies; press onto sides of cake, standing vertically and alternating chocolate and vanilla cookies. Trim cookies with serrated knife to make neat fit at corners of window box. Tie string around box to keep cookies in place until frosting hardens.
Sprinkle ground chocolate cookie crumbs over top of cake, for dirt.
Arrange all flowers and leaves in Window Box Cake. Makes 16 servings.