These are legion. I’m only reproducing a few here.

How I make a Boulevardier
2 rye
1/2 Campari
1/2 Cocchi Torino vermouth
stir, up, one big rock, orange twist.

Doug Ford is right, the best riffs off the Boulevardier are the 1794 and 1795:

1794

  • 2 oz Rye
  • 1 oz Sweet Vermouth (M&R Rosso)
  • 1 oz Campari
  • 1 dropper Bittermen’s Xocolatl Mole bitters
  • orange or lemon twist for garnish (optional)

1795
Ted Gallagher, Craigie on Main, Boston, 2010

  • 1–1½ oz rye
  • ½ oz Carpano Antica sweet vermouth
  • ½ oz Punt e Mes
  • ½ oz Campari
  • ½ oz Aperol
  • 1 dropper Bittermens Xocolatl Mole bitters

but I can’t leave well enough alone…

Eigengroni
2 gin (Botanist)
1 Carpano Italian Vermouth
1 Amaro Nonino
orange twist

Negrotrar
1 Campari
1 Cynar
1 Green Chartreuse
2 shakes celery bitters
stir, strain. No garnish, although I’m tempted to add a sprig of dill.

Cynevardier
1 rye whiskey
1 cynar
1 campari
Rub glass with kumquat or orange.
Stir with ice, serve up or on the rocks.
c/f Doug Ford’s masterly 1794 post for a bunch more variants and historical notes

The Unholy Mountain
2 gin
1 campari
1 cynar
1/2 green chartreuse
4 dashes Peychaud’s Bitters
2 dashes orange bitters
stir, strain, drink.

abusing the formula
2 rye
1/2 Zucca
1/2 Campari
2 dashes green Chartreuse
2 dashes Angostura
1 dropper xocolatl bitters
1 dropper st george absinthe
orange twist
Stir, rocks.

This Machine Annoys Geert Wilders
2oz barrel-aged Brokers gin
1/2 oz Cocchi Torino vermouth
2 dashes orange bitters
1/2 spoon Strega
Stir, up, twist.

Regina
1 Campari
3/4 Rye
3/4 Mezcal
1/2 Cocchi Torino vermouth
1/2 Vermut (Miro)
kumquat for garnish. Or, failing that, orange twist.