"Improving" a cocktail does not necessarily mean making it better. The term simply means something was added to the traditional combination of spirit, sugar, water, bitters associated with an old fashioned style cocktail. In this case, Charlotte "improves" this tequila old fashioned by adding Maraschino and two types of bitters.
2 dashes Angostura Aromatic Bitters
2 dashes Peychaud's Bitters
1 bar spoon simple syrup
1 bar spoon Maraschino liqueur
Stir all ingredients with ice.
Strain into a pre-chilled cocktail coupe.
Garnish with a lemon twist.