For Plain Custard Base: In a 4-quart saucepan, combine milk, cream, and half of sugar. Set over high heat, and cook, stirring occasionally, until mixture comes to a boil, about 5 minutes. Meanwhile, in a medium bowl, whisk yolks and remaining sugar until smooth, heavy, and pale yellow, about 30 seconds. When cream mixture just comes to a boil, whisk, remove from heat, and, in a slow stream, pour half of cream mixture over yolksugar mixture, whisking constantly until blended. Return pan to stovetop over low heat. Whisking constantly, stream yolk-cream mixture back into pan.
With a wooden spoon, continue stirring until mixture registers 165 to 180 degrees on an instant-read thermometer, about 2 minutes. Do not heat above 180 degrees, or eggs in base will scramble. Mixture should be slightly thickened and coat back of spoon, with steam rising, but not boiling. (If you blow on the back of the spoon and the mixture ripples, you’ve got the right consistency.) Pour base into a clean airtight container and refrigerate for 12 to 24 hours before using. Use base within 3 to 5 days.
For Ice Cream: In a heavy saucepan, combine blueberries, sugar, acai juice, and lemon juice and cook over low heat, stirring occasionally, until berries burst and mix thickens slightly, 15 to 20 minutes.
Strain mixture through a fine-mesh sieve into a bowl to remove seeds. Discard solids left in sieve. Set syrup aside to cool.
Process base in an ice cream maker according to manufacturer’s instructions. Transfer to a bowl and fold in blueberry syrup to make an attractive swirl. Fold in goji berries.
Scrape into an airtight storage container. Freeze for a minimum of 2 hours before serving.