Step 1

Place the sugar, flour, cornstarch and salt in a small pot and whisk to combine.

Step 2

Gradually whisk in the milk, ensuring there are no lumps.

Step 3

Place over medium-low heat and cook, stirring often, until thickened.

Step 4

Pass the mixture through a fine mesh strainer, and into a heat-safe container.

Step 5

Refrigerate for 2 hours.

Step 6

Place the cooled mixture in a large mixing bowl, along with the lemon juice and vanilla, and whip on medium-high speed.

Step 7

Whip in the cream cheese, a tablespoon at a time, allowing each addition to fully incorporate before adding the next.

Step 8

Whip in the butter, a tablespoon at a time, allowing each addition to fully incorporate before adding the next.

Step 9

Turn the mixer up to high speed, and continue to whip until very fluffy.