The secret to this ice cream's rich coffee flavor lies in the espresso powder.
Prepare an ice-water bath. Stir together espresso powder and hot water in a small bowl. In a medium bowl, whisk together 1/4 cup sugar and egg yolks.
Combine cream, milk, remaining 1/4 cup sugar, and salt in a medium saucepan over medium heat. Scrape vanilla seeds into pan; add pod. Bring mixture just to a simmer. While whisking constantly, slowly ladle about 1/4 cup hot milk mixture into egg yolk mixture. Continue adding remaining milk mixture, whisking constantly, about 1/4 cup at a time.
Pour mixture back into saucepan; cook over low heat, stirring constantly, until mixture is thick enough to coat the back of a spoon, 3 to 5 minutes. (The custard should retain a line drawn with your finger across the back of a spoon.) Stir in espresso mixture. Strain through a fine sieve into a medium bowl set in the prepared ice-water bath. Stir occasionally until cool. Cover with plastic wrap, and refrigerate at least 1 hour and preferably overnight.
Pour mixture into ice cream maker. Process according to manufacturer's instructions. The ice cream is ready once it holds its shape when the machine stops but sags slightly. Transfer to an airtight container and freeze.