Be sure to let the frozen yogurt sit and thaw slightly – for 5 or 10 minutes, say – before you scoop and serve it. The texture is best that way. Also, I use Brown Cow brand yogurt.
ONE DAY AHEAD:
To make the blueberry sauce, put the blueberries and sugar in a small saucepan, stir to mix, and bring to a boil over medium heat. Reduce the heat and simmer, stirring occasionally, until the berries are very tender and the sauce is slightly thickened, about 8 minutes. Remove from the heat, and let cool. Refrigerate until cold before using.
To begin the frozen yogurt base, place a sieve over a bowl, and line it with two layers of cheesecloth. Pour the yogurt into the sieve, cover with plastic wrap, and refrigerate for 6 to 8 hours to drain. Discard the liquid, and measure out 1 ¼ cups drained yogurt. Set aside. [You will have some drained yogurt left over, and it’s delicious for breakfast.]
THE DAY OF:
First, make the lemon syrup. Using a vegetable peeler, remove the zest from 1 lemon in large strips; reserve for the frozen yogurt base. Then juice enough of the lemons to yield ½ cup. Combine the lemon juice and sugar in a small saucepan, and bring to a boil over medium-high heat, stirring to dissolve the sugar. Remove from the heat and let cool.
Put 2 tablespoons of the milk in a small bowl, add the cornstarch, and whisk until you have a smooth slurry.
In a medium bowl, whisk the cream cheese until smooth.
Fill a large bowl with ice and a little bit of cold water.
Combine the remaining milk, the cream, sugar, corn syrup, and strips of lemon zest in a 4-quart saucepan. Bring to a rolling boil over medium-high heat, and boil for exactly 4 minutes. Remove from the heat, and slowly whisk in the cornstarch slurry. Return the pan to the heat, and continue to cook, stirring with a heatproof spatula, until slightly thickened, about 1 minute more. Remove from the heat, and then gradually whisk the hot milk mixture into the cream cheese until smooth. Add the reserved 1 ¼ cups drained yogurt and the lemon syrup. Whisk until smooth.
Pour the mixture into a 1-gallon Ziploc freezer bag, and submerge the sealed bag in the ice bath. Let stand, adding more ice as necessary, until cold, about 30 minutes.
Remove the strips of lemon zest from the frozen yogurt base. (I did this by pouring it through a mesh strainer directly into the ice cream machine.) Pour the mixture into the canister of an ice cream machine, and spin until thick and creamy.
Pack the frozen yogurt into a storage container, alternating spoonfuls of yogurt with spoonfuls of the blueberry sauce, and do not mix them. (You’re basically creating pockets and splotches of sauce within the frozen yogurt.) Press a sheet of parchment directly against the surface, and seal with an airtight lid. Freeze until firm, at least 4 hours.
Yield: 1 generous quart