Spoon 1 tsp cocoa powder and 1 tsp sugar into each of two dry 110 ml / 4 oz heat-safe glasses.
Pull one 30 ml espresso shot directly into each glass, or divide 60 ml of strong moka pot coffee between them. Stir with a teaspoon for 10 seconds until the cocoa dissolves into a glossy, thin paste.
Pour the cold milk into a 1 L / 1-quart saucepan and warm it over low heat for 2 to 3 minutes, stirring, until it reaches 60 C / 140 F and steams without simmering. Pour it into a 250 ml / 1-cup tall jug.
Froth the milk with a handheld frother, keeping the head just under the surface, for 20 to 30 seconds until the volume roughly doubles and it looks like wet paint. With a steam wand, steam it in a small pitcher to the same temperature. Tap the jug on the counter to knock out large bubbles.
Hold a teaspoon just above each glass and slide the foam over the espresso until it reaches the rim. Dust each glass with 1/4 tsp cocoa through a small sieve and serve within 5 minutes.