Heat the oven to 170C/325F/gas mark 3. Spread the almonds out on a baking sheet and toast in the oven for 5-10 minutes, keeping a close eye on them, until lightly golden brown.
Put the cream, milk, vanilla, half of the sugar and the toasted almonds in a saucepan. Heat over a low heat, stirring occasionally, until almost boiling, then remove from the heat and set aside for 30 minutes for the almonds to infuse.
Put the egg yolks into a bowl with the other half of the sugar and whisk until smooth and pale. Re-heat the almond and milk mixture until steaming then gradually pour it into the egg yolks while whisking constantly.
Return the mixture to the pan and place over a low heat. Stir constantly until thickened, do not let it boil! The mixture should reach 75C and not exceed 80C. Pour the custard through a sieve into a bowl; press on the almonds left in the sieve to extract as much flavour as possible then discard them.
Add a couple of drops of almond extract if you think that the almond flavour isn't strong enough then cover the bowl and allow to cool. Refrigerate until completely cold then churn in an ice cream maker according to the manufacturer's instructions.