Fill the bottom chamber of a 6-cup moka pot with the water to just below the safety valve.
Fill the filter basket with the finely ground dark roast coffee, level it without tamping, and assemble the pot.
Set over medium heat with the lid open. As the first drops of coffee appear, quickly pour about 1 tablespoon into a heatproof cup containing the granulated sugar, then return the pot to the heat.
Brew until a steady stream forms and you hear a gentle gurgle, 3–5 minutes total; remove from heat just as it begins to sputter to avoid bitterness.
Vigorously beat the sugar and captured coffee with a spoon until the mixture turns pale and thick, 1–2 minutes, forming espumita.
Slowly pour the remaining brewed coffee into the sugar paste while stirring constantly until fully dissolved and a thick foam rises on top.
Let stand 15–30 seconds for the foam to settle, then pour into demitasse cups and serve immediately (or serve as a colada with small cups alongside).