Prep the strawberries. If using frozen, thaw and cook them gently until soft enough to blend. If using fresh, wash and remove the stems.
1 cup strawberry puree
Puree and strain. Blend the strawberries into a smooth puree, then strain through a fine mesh sieve. The finer the strainer, the silkier the cream.
Heat the milk. Add the milk, sugar, and vanilla extract to a saucepan. Heat over medium until it just begins to steam and is close to a boil.
1 cup milk, 7 tbsp sugar, 1 tsp vanilla extract
Prepare the yolk mixture. While the milk heats, whisk the egg yolks, strawberry puree, and cornstarch in a large heatproof bowl until smooth. Make sure the puree isn't hot before adding the yolks.
4 egg yolks, 5 tbsp cornstarch
Temper the eggs. Remove the hot milk from the heat. Slowly pour it into the yolk mixture in a steady stream, whisking constantly.
Cook until thick and bubbling. Return the mixture to the saucepan over medium heat, whisking constantly. Once it thickens and begins to bubble, reduce the heat and whisk for another 30–60 seconds to cook out the raw starch taste.
Whisk off the heat. Remove from the heat and continue whisking for 30–60 seconds — this loosens the cream slightly so it sets smooth rather than overly stiff.
Add the butter. Stir in the butter until fully melted and smooth.
2 tbsp unsalted butter
Taste and adjust. Add lemon juice if the cream needs a bit of acidity, or more sugar or a pinch of salt to taste.
1 tbsp lemon juice
Cool. Transfer to a bowl and press plastic wrap directly onto the surface to prevent a skin from forming. Cool at room temperature, then refrigerate until needed.