To make the ice cream, combine the milk and cream in a large saucepan. Scrape out the small black seeds from the vanilla bean and add both the bean and seeds. Heat until almost boiling. Keep hot over low heat, stirring occasionally.
In a large bowl, whisk together the egg yolks and sugar until thick and pale. Little by little, add the hot milk and cream mixture, whisking constantly. The mixture should thicken into a smooth, thick custard that leaves a thick film on the back of a spoon; if the custard is still too thin, return it to the pan and heat, whisking continuously, until thickened (it should reach about 175°F on a candy or instant-read thermometer). Do not overheat or the custard will curdle. Let the custard cool until lukewarm, then chill in the refrigerator (or freezer) until very cold.
Freeze in an ice cream machine according to the manufacturer’s instructions, transfer to an airtight container, and freeze. Or, if you do not have an ice cream machine, transfer the custard to a container with a tight-fitting lid and place the container in the freezer. Take the custard out every 20 minutes and stir it with a spatula, making sure to scrape the frozen crystals from the sides of the container, until the ice cream is frozen and it gets difficult to move the spatula around. Return to the freezer.
To make the almond brittle, cover a table with parchment paper. Heat a large nonstick skillet over medium-high heat. Add the sugar and butter and cook until the sugar has melted and you have a golden brown caramel. Add the almonds and hazelnuts, stirring to coat. Immediately pour the caramel onto the parchment paper and spread it out with a spatula until 1/4 inch thick. Let cool.
Coarsely chop the almond brittle. Mix most of the almond brittle into the ice cream and return it to the freezer until ready to serve. Reserve the remaining almond brittle for garnish.