In a medium bowl, mix sugar, cornstarch, and salt together, then add it 1/2 cup milk and whisk smooth.

In a small bowl mix egg yolks, then add them to the mixture and blend until creamy.

In a small saucepan, heat the remaining milk to a simmer. (small bubbles around the edge, not a full boil)

Slowly pour the heated milk into the egg mixture while whisking constantly. Allowing the egg mixture to slowly heat to temper the eggs so they don't scramble.

Place the mixture into the saucepan and heat on medium-low heat whisking until it comes to a boil and is smooth and creamy. This takes about 5 minutes.

Remove pan from heat, stir in butter and lemon emulsion.

Place pastry cream into a bowl and cover with plastic wrap, allow it to cool for 15 minutes, then put it into the refrigerator to chill