Using a citrus juicer, extract the juice from the grapefruits, keeping the peels. Place half the peel from a grapefruit into a mortar along with 2 sprigs of mint and crush with the pestle. Discard the peel and put the crushed mint into a cocktail shaker. Add ice and the Esperanza and shake until combined. Rub the rim of a tumbler with a little honey and dip into the salt. Fill the glass with crushed ice and pour over the Esperanza mixture. Top with the ruby grapefruit juice and garnish with a sprig of mint.