Line a jelly-roll pan with parchment paper and lightly grease the paper with oil.
Cut a second piece of parchment paper the same size, oil it lightly and set aside.
Sprinkle the gelatin over half a cup of cold water and briefly stir to completely moisten it and break up any clumps.
Let the gelatin sit in the cold water until it swells and softens (10 to15 minutes).
Combine the sugar and corn syrup with the remaining half cup of water in a heavy bottomed saucepan and stir to moisten the sugar.
Cook over high heat, stirring constantly until the mixture comes to a boil.
Immediately stop stirring and skim the surface to remove any scum that has risen to the top.
Continue to cook over high heat, occasionally brushing down the side of the pan using a pastry brush and water, until the mixture registers 242 degrees on a candy thermometer.
Remove the mixture from the heat and let cool to approximately 210 degrees.
Place the gelatin in a heatproof-bowl over a pan of simmering water.
Stir constantly until the mixture is clear and liquid.
Mix the liqueur into the dissolved gelatin.
Mix the gelatin into the cooked sugar mixture and transfer to a mixing bowl.
Whip the mixture with an electric mixer on high-speed until medium peaks form (about eight to 10 minutes).
Quickly spread the mixture onto the lined pan.
Top the mixture with the prepared oiled parchment paper and use a rolling pin to smooth it into an even slab.
Place the pan in the freezer for at least eight and up to 24 hours before taking the slab out of the pan.
After chilling, gently peel off the paper from one side.
Lightly dust the marshmallow slab with some cornstarch.
Flip the slab over and gently peel off the parchment paper from the other side.
Dust with cornstarch.
Cut the marshmallows to your desired size.
You can store the marshmallows in an airtight container in the freezer for up to four months.