Muddle the large ripe raspberries in a Boston shaker to release the juices.
Add Arrogante Reposado tequila, freshly squeezed lemon juice, orange bitters (such as Angostura Orange Bitters or Regan's Orange Bitters), agave nectar and, if using, a small pinch of red pepper flakes.
Add a handful of ice to the shaker and shake vigorously for 15 to 20 seconds until the drink is thoroughly chilled and the frost starts forming on the outside of the shaker.
Strain the cocktail through a fine mesh strainer into a chilled coupe or martini glass. Garnish the drink with a lemon twist.