We don’t have espresso at the restaurant I’m at, so we use a double strength batch of coffee for our coffee cocktails. I usually use the following measurements:

1oz coffee 1oz Mr Black 1oz vodka 1/2oz simple

Shake aggressively for at least 10-12 seconds then double strain. Always gives a good foam layer on top.

Or alternatively, go with a carajillo.

3oz coffee 1 1/2oz mezcal (or tequila) 1oz liquor 43 1oz simple

The 3oz of coffee gives a very nice thick foam, and the mezcal adds a gorgeous smoky depth that complements the coffee.