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.
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.
Cook the Pudding. As the milk mixture nears a boil, stir in the starch slurry. Continue stirring on low heat until it thickens. Then turn off the heat.
Chill. Cover the pudding's surface with plastic wrap to prevent a skin from forming. Let it cool to room temperature, about 60 to 90 minutes.
Prep the Butter. Cube the butter and bring it to room temperature, so it's ready when the pudding is.
Make Pastry Cream. Whip the room-temperature butter, starting slow and increasing to high speed until aerated. Gradually mix in the cooled pudding, one tablespoon at a time, until well incorporated and creamy.
Taste Test. Adjust the sweetness with powdered sugar if needed.
Refrigerate. Chill the cream for at least an hour before using, allowing it to thicken and set.