In a jigger, measure out the absinthe, then drop in the sugar cube to allow it to partially absorb the liquor.
Meanwhile, in a rocks glass, muddle the Thai basil and yuzu. Add the tequila, then fill the glass halfway with ice and stir with a bar spoon until well chilled.
Place the absinthe spoon on top of the glass and place the absinthe-soaked sugar cube on top of the spoon. Using a lighter or torch, ignite the sugar. As it begins to caramelize and dissolve into the drink, pour the remaining absinthe over the flame to fuel the fire. Once the sugar is fully melted, use the absinthe spoon to stir the residue into the cocktail. Fill the rest of the glass with ice, stir and garnish with a sprig of Thai basil.