Very tasty with Mr. Black coffee liqueur and cool espresso (I used Nescafe Gold instant).

Coffee liqueurs run the gamut on sweetness from the very dry St. George Nola to the slightly sweet Mr. Black to the sweet Borghetti and Grind to the very sweet Kahlua, and thus people may want to adjust the amount of espresso to balance.

I prefer this as an equal parts cocktail omitting the sweet vermouth:
1 oz London dry gin
1 oz Campari
1 oz coffee liqueur (Mr. Black for me)

I make mine less sweet and more Irish as an equal parts cocktail with Irish whiskey and coffee liqueur:

1 oz Irish whiskey
1 oz coffee liqueur
1 oz Irish cream
1 oz espresso

You can also try blanco or reposado tequila and light or dark rum in place of the whiskey for interesting variations.

Absolutely. Difford's calls it an Irish Chocolate Oranj'tini, but I call it a Scrambled B-52 cocktail.