Cut up almond paste in to small chunks and place in a stand mixer bowl (a hand held one does not work well with this recipe; the motor is not strong enough). Attach paddle (or dough hook if making the larger amount) and pulse (turn on and off in slightly fast succession) on low speed until the paste is broken up, for about 35 - 45 seconds.

Add half of the powdered sugar and continue to pulse while slowly pouring in the vanilla, rum and corn syrup  until the dough come together and sticks to paddle or dough hook.

Place remaining confectioner's sugar in a mound on the countertop. Put dough in the center of it and knead in all of it. Marzipan should have a fine smooth texture and feel like heavy pie dough when done. 

If it sticks to the countertop, it needs a bit more confectioner's sugar. Flatten the marzipan and lightly sprinkle more sugar on top. Knead it before deciding if you need more. Do not make it too stiff. Marzipan can be softened by adding small amounts of light corn syrup to it.

It can then be tinted, if applicable.

Once made, store marzipan to be used for covering cakes with and for molding, in a plastic bag placed in an airtight container for 1 month in the refrigerator. It can also be frozen for about 6 months. Thaw in refrigerator overnight and then warm to room temperature before using. (I like to use mine at slightly cooler than room temperature). Knead to soften, but don't let it get oily. If it does, press paper towels on its surface to absorb.

Once tinted, molded and dried, store marzipan in an airtight container.

If at any time the marzipan smells rancid, toss -- because it is made from nuts whose oils are perishable. 

Toba Garrett, Cookie Decorating, Delicious Decorating for Any Occasion.