Add milk to a small saucepan. Scald over medium-high heat (until small bubbles appear at the edges).
Meanwhile, in a mixing bowl, whisk together sugar, flour, and egg yolks until you have a thick paste.
Slowly pour the hot milk into the egg mixture, whisking the whole time.
Return the whole mixture to the pan over the fire, whisking the entire time. Stop mixing every few seconds to see if mixture has come to a boil. Once mixture is boiling, whisk 10 more seconds. The cream should be thick at this point.
Pour the cream through a fine sieve to get rid of any possible lumps. Place in a storage container or bowl and cover with plastic wrap. The plastic should be touching the surface of the cream so no skin forms. Chill in the refrigerator for at least 4 hours.
Once ready to fill donuts, whip the heavy cream to stiff peaks and fold into pastry cream.