Pour 1 tsp of absinthe into a rocks glass. Swish the absinthe along the bottom and sides of the glass, be sure to coat every part of the interior of the glass. Dispose of the excess absinthe.
Fill the glass with ice cubes and add all the cocktail ingredients, except for the absinthe (which you've already used for the rinse) and orange peel. Stir for about 30 seconds, then strain through a Hawthorne strainer into a rocks glass filled with ice. Roll up the orange peel and squeeze it above the cocktail to express the citrus oils before placing it in the glass as garnish.
Place all the ingredients for the rooibos tea syrup in a small saucepan with 1 cup of water. Bring to a simmer over medium heat and simmer for about 20 minutes, until the liquid has reduced by a third. Remove from the heat and set aside to cool before straining through a fine-mesh sieve and transferring to a sealed bottle or jar. The aromatics can be discarded. Keep in the fridge until ready to use. Rooibos tea syrup will keep in the fridge for up to 3 weeks.
Recipe adapted from: Ottolenghi, Yotam and Ramael Scully. "Rooibos Old Fashioned." Nopi. London: Appetite by Random House, 2015.