2 cups fresh or frozen raspberries
1/4-1/2 cup sugar (depending on sweetness of berries)
For custard base:
2 cups heavy cream
6 egg yolks (4 or 5 will work fine, but no less than 4)
3/4 cup sugar
1 tsp. vanilla, optional
2 cups cold buttermilk
Place the berries in a bowl and sprinkle with the sugar. Let sit for several hours or overnight.
Mash berries with a potato masher and strain through a fine mesh sieve over a clean bowl. Press the berry solids to extract as much puree as possible. Refrigerate until ready to use.
For the custard:
Heat the heavy cream in a heavy-bottomed saucepan until very hot, but do not boil.
Whisk the egg yolks, sugar, and salt until thick and the sugar has started to dissolve. Slowly add the cream in a thin, steady stream while whisking. Pour back into the saucepan.
Cook the custard over low to medium heat, careful not to boil, until it becomes thick and coats the back of the spoon. (The temperature should cook to 160 degrees F, but no higher or it may curdle.) Strain the custard over a clean bowl through a fine mesh sieve.
Add the cold berry puree and buttermilk; whisk well. Taste the custard and adjust for sweetness, if necessary. Refrigerate the custard until it is very cold.
Freeze in an ice cream machine according to manufacturer instructions.