Put sansho peppercorns and cheap vodka in a sealed bottle and let them infuse for at least a week. Gently shake or swirl the bottle every day and store away from intense light or heat.
To make the cocktail, pour all ingredients into a cocktail shaker with ice. Shake well and strain into martini or coupe glasses, garnish with lemon slices.
Depending on how long your sansho vodka has been infusing and how strong a sansho flavor you like, you can vary the ratio of infused to plain vodka in the cocktail. Optional: Add a few drops of yellow food coloring for visual appeal.