Pudding Prep. Reserve one-third of the milk. Pour the rest into a pot with sugar, egg yolks, and lemon juice, and stir well. Add the raspberries, mix again, and heat until near boiling, stirring often. Then simmer for 2 to 3 minutes until the raspberries break down.
1.25 cup milk, 4 egg yolks, 7 tablespoons sugar, 1 tablespoon lemon juice, 8.5 oz raspberries
Blend and Strain. Remove from heat, blend with an immersion blender, and strain through a fine mesh sieve. Rinse the pot to remove seeds and raspberry bits, then return the strained mixture to it and heat until almost boiling.
Prepare Starch Slurry. Mix the reserved milk with starch in a separate bowl until smooth.
4 tablespoons cornstarch
Add Cornstarch Slurry. As the milk mixture goes from warm to hot a boil, stir in the starch slurry.
Thicken Cream. Cook on low/medium heat, stirring constantly, until it thickens and bubbles (2-4 minutes).
Off Heat Whisking. Once thick, remove from heat and whisk for another minute.
Add Butter. Stir butter into the mixture until melted and integrated.
2 tablespoons unsalted butter
Taste and Adjust. Taste the cream and add more sugar, salt, or vanilla if needed.
(Optional) Strain. If you're not sure your pastry cream is nice and smooth, strain it.
Chill. Cover with plastic wrap touching the cream's surface to avoid crust formation. Cool at room temperature for 30 minutes, then refrigerate.
Final Whisk. Before use, "chop" the chilled cream with a fork, then whisk until smooth. It's now ready for piping.