Combine 1/4 cup of the sugar, 1/2 cup of the milk and all of the cream in a heavy-bottomed medium saucepan. Bring just to the simmering point over low heat.
In the meantime, combine the remaining cup of milk, 3/4 cup of sugar, cocoa and egg yolks in a large bowl and whisk until just blended.
While gently whisking the cocoa mixture, drizzle the hot cream mixture into it so that it is gradually warmed up. Return everything to the saucepan and cook while stirring with the whisk. Make sure that you are constantly scraping the whisk across the bottom of the pan so the custard does not scorch. The custard is done when it has thickened slightly and can evenly coat the back of a spoon. It takes about 2 minutes total, after everything is returned to the saucepan. Do not let it come to a boil. Strain the custard through a fine sieve into a bowl and nestle it into a large bowl of ice. Let cool, stirring occasionally. Transfer custard to an ice cream machine and freeze according to the manufacturer's instructions. Put the finished cream in a storage container and freeze until firm.