Add <quantity>2<unit>cups</unit></quantity> of milk and <quantity>120<unit>g</unit></quantity> of egg yolk to a saucepan

Add <quantity>50<unit>g</unit></quantity> of cornstarch and a mix of <quantity>150<unit>g</unit></quantity> of sugar and the seeds from <quantity>1</quantity> vanilla bean

Turn the heat to medium and whisk continuously until the cornstarch is activated and the mixture is very thick

Pass the custard base through a fine sieve

Place it in the refrigerator until it is slightly chilled

Pour <quantity>½<unit>cup</unit></quantity> of heavy cream into a stand mixer bowl. Whip with a whisk attachment on medium speed until soft peaks are formed

Add the whipped cream to the custard base and mix to incorporate

Season with the zest of <quantity>1</quantity> lemon and mix

Transfer to a piping bag with a tip. The custard filling is ready

Enjoy! You can use it on doughnuts or any of your favorite desserts.