To make the simple syrup, heat the water, sugar and coriander seeds over medium heat in a heavy-bottom saucepan until the mixture just comes to a boil. Remove from heat, cover and refrigerate for at least one hour. The longer the syrup sits, the more the flavor will develop. Drain the liquid away from seeds and set aside (store what you don’t use in a mason jar for later).
To mix the drink, combine the vermouth, orange liquor, gin, simple syrup and bitters over ice in a cocktail shaker. Shake well. Pour into cocktail glasses. Pour ginger beer on top of each cocktail as a floater. Garnish with orange slices. Chee