Prepare like an old fashioned. Muddle the sugar cube with the Fernet in a chilled old fashioned glass. Press out the oils on the grapefruit zest with your muddler. Add the rum, tequila and Chase Elderflower liqueur. Add ice. Stir well.
This cocktail does not work as well with St Germain. If St Germain is all you can find, cut the Fernet down slightly.