Bring milk and 1/2 of the sugar to a boil. Reduce heat to low.
In a bowl, whisk together egg yolks, egg, the remaining sugar, corn starch and vanilla.
Add 1/2 cup of the hot milk to the egg mixture and whisk to combine. Then pour mixture into the sauce pan with the milk. Cook over medium-low heat, whisking constantly. Cook for 3-5 minutes, until thickened. Add butter.
Strain through a fine mesh sieve. Let it cool to room temperature.
Sprinkle gelatin over cold water. Let it sit for 5-10 minutes, until it blooms.
Warm up gelatin over a double boiler or for 10-20 seconds in the microwave.
Pour gelatin over the custard, whisking constantly to combine.
Beat heavy cream until soft to medium peaks (5-6 minutes on medium speed). Do not overbeat, it will ruin the texture of the pastry cream.
For soft-medium peaks - if you take your whisk out of the bowl, more of the cream will cling to the whisk but any peaks will quickly soften and curl.
Fold into the custard.
Pour into glasses, or use to fill cakes, eclairs or tarts.