To make the cookies: In a large mixing bowl (or stand mixer), cream together butter, Crisco, sugar, powdered sugar and milk. Add egg and mix until well combined. In a separate bowl, combine flour, baking soda, cream of tartar and salt. Add the flour mixture to the butter mixture and mix until smooth and well combined. Roll the dough into 1-inch balls and place on a greased cookie sheet. Spray the bottom of a glass or measuring cup with cooking spray and then dip into a plate coated with a thin layer of sugar. Press each ball down slightly. You don’t want to smash them too thin because these cookies are best when they are thick and soft. Bake for 8-9 minutes at 350 degrees F. Cool cookies completely.
To make the frosting: Mix butter, vanilla and milk for 30 seconds with a mixer. Add in powdered sugar 1 cup at a time until you reach your desired consistency.
Divide the frosting into two bowls with one with slightly more frosting than the other. Color the bowl with more frosting red. (I used about 1/8th tsp coloring.) Color the other bowl of frosting green. (I used 5-6 drops.) Transfer the green frosting to a piping bag with a medium sized round tip.
Frost a circle on the inner portion of each cookie with red frosting using a spreading knife. Pipe green frosting around the edges of the red frosting. Sprinkle with chocolate jimmies. I loved the cookies both as round "slices" of watermelon, and cut in half too!
Store cookies in a closed container.