1) If you are going to pour the marshmallows like fudge and cut into pieces, butter a pan (8x8β for tall marshmallows, 9x13β for shorter ones). I tend to use a jelly roll pan lined with waxed paper. I coat the waxed paper with vegetable oil spray.
Alternatively, for Easter eggs fill four 9x13 pans with flour (about 2 inches deep) and hollow out egg shapes using the back of a spoon or a plastic Easter egg (or a real egg, but I'd wash it first).
2) Dissolve gelatin in cold water, set aside. Put 1/2 cup Karo, the hot water and sugar into a 2-qt pan. Stir with a wooden spoon while heating (medium high heat) until it boils. Remove spoon; wash down sides with pastry brush dipped in warm water. Clip on candy thermometer and cook until soft ball stage (238F at sea level, 230 at my house).
3) Remove from heat and stir in remaining corn syrup. Pour into medium-size bowl. Using an electric mixer at high speed, beat hot syrup, adding gelatin mixture 1 tablespoon at a time. Continue beating until all gelatin is incorporated, candy is thick and has cooled to lukewarm, about 10 minutes. Stir in vanilla. Pour or spoon into prepared pan(s).
4) Cool 3 hours or until marshmallow is firm enough to cut with an oiled cookie cutter or a knife dipped in hot water. Cut marshmallows into desired size. Roll cut marshmallows in powdered sugar to prevent sticking, or dip into melted chocolate.