Marshmallow Slab:

Line an 8×8-inch square tin with parchment, and coat generously with oil. (See notes above for the best lining technique!) Set aside until ready to use.

Pour 120 milliliters (1/2 cup) red wine into a large mixing bowl (if using a hand mixer), or the bowl of a stand mixer. Sprinkle the gelatine evenly over the water, and set bowl aside.

Combine sugar, corn syrup, remaining wine, and salt in a large saucepan. Bring mixture to a boil and boil, without stirring, until syrup reaches 240° F / 116° C on a candy thermometer.

Carefully pour hot syrup into gelatine mixture. Mix on high speed until very thick, about 10 minutes.

Using an oiled spatula, scrape mixture into prepared tin. Loosely cover with a well-oiled sheet of aluminum foil. Allow to set until firm, at least 4 hours or overnight.

Cutting and Coating:

In a small bowl, combine potato starch and icing sugar.

Cover a large chopping board with a sheet of parchment, and dust generously with starch-sugar mixture.

Turn marshmallow slab out onto dusted surface. Sift top of slab with more of the starch-sugar mixture.

With a sharp, well-oiled knife, cut slab into squares. (If desired, a metal biscuit cutter may be used to cut the marshmallows into fun shapes.)

Toss cut marshmallows in additional starch-sugar mixture, making sure all sides are coated.

Marshmallows will keep well for several weeks when stored in an airtight container at room temperature.