Cut the lemons in half lengthwise and remove the flesh with a spoon. Put the lemon halves cavity-side up in a muffin tin.
Add the gelatin dessert powder to the boiling water and whisk well to combine. Add the vodka and whisk to incorporate. Transfer to a cup measure and carefully fill the lemon cavities.
Chill for 4 hours, slice and serve.
For white shots, substitute 1/4 cup sugar, one 1/4-ounce package plain gelatin and the juice of 1 lemon for the flavored gelatin and follow the same procedure.