In a medium saucepan, combine strawberries, lemon juice, and sugar. Cook over medium heat until juice simmers and berries have softened.
Transfer mixture to the bowl of a food processor. Pulse the mixture until smooth. Using a fine mesh strainer, strain the berry mixture, returning the juice to the saucepan. You should have about ½ cup juice.
Cook juice over medium-low heat until reduced to about ¼ cup. Remove from the heat and set aside.
For Pastry Cream
In a small bowl, combine cornstarch (see note), ¼ cup sugar, and egg yolks. Whisk until smooth. Set aside.
In a medium saucepan, combine milk, cream, ¼ cup sugar, vanilla bean paste (see note), and salt. Cook mixture oven over medium heat until it reaches a boil. Remove from the heat.
Using a heatproof measuring cup, pour about one cup of the hot cream mixture into the egg mixture, whisking constantly. This step tempers the eggs. Once the mixture is combined, pour the tempered egg mixture into the saucepan with the cream mixture.
Cook pastry cream over medium heat, stirring constantly, until it just starts to boil. Mixture will thicken as it cooks.
Remove the pan from the heat, and whisk the pastry cream until it cools slightly. Mix in butter and strawberry reduction until fully incorporated.
Pour the pastry cream into a medium bowl. Cover with plastic, pressing the wrap to the surface of the pastry cream to ensure it doesn't form a skin. Refrigerate until completely cooled, at least 3 hours.
The pastry cream can be stored in the refrigerator, with plastic wrap pressed to the entire surface, for up to four days.If you'd like to freeze the pastry cream for longer storage, switch out the cornstarch for flour as a thickener. While I prefer the cornstarch, it will not freeze well. The flour will thicken the pastry cream, and will freeze and thaw without compromising the texture. The pastry cream will keep in an airtight container for up to a month.Enjoy!
Notes
Note on cornstarch: If you'd like to freeze the pastry cream for longer storage, switch out the cornstarch for flour as a thickener. While I prefer the cornstarch, it will not freeze well. The flour will thicken the pastry cream, and will freeze and thaw without compromising the texture. Note on vanilla bean paste: I always have vanilla bean paste on-hand and prefer it's flavor to extract, so that's what I use. If you don't want to use vanilla bean paste, you can add vanilla extract at the end of cooking the pastry cream. Note on food coloring: While not necessary, as the pastry cream will still have a pretty pink color without it, you can add a few drops of pink food coloring if you'd like it to be more vibrant.