In the stainless steel bowl (note: you’ll use this bowl as a makeshift double boiler later) of your stand up mixer, add the softened butter, sugar and vanilla extract and beat until light and fluffy, about 2 minutes.
Add the egg yolks, beat the mixture together, and then add the eggs, beating again until they’re well incorporated. Stir in the salt, and then in a few batches, add the lemon juice. The mixture will look curdled and you might scratch your head and wonder if it’s right, it will be!
Fill a medium size saucepan with a few inches of water. Bring the water to a simmer and place your stainless steel bowl of curd mixture on top. Stir constantly, and heat the curd slowly enough that the sugar has time to dissolve and the whole mess becomes cohesive; this step usually takes about 10 minutes. Pull the curd from the head when it is just thick enough to coat your spoon and until a thermometer reads 166 F. The curd will thicken as it cools.
I ran the curd through a sieve into a chilled bowl just to ensure it was silky smooth. I highly recommend this step! Transfer the curd to a container a keep in the fridge, in an airtight container, for up to a week.
When you’re ready to use the curd, add the heavy cream to a bowl. Whisk aggressively until medium peaks form, about 5 minutes. You can also use an electric hand mixer or stand-up mixer. Gently fold the whipped cream into the lemon curd until completely cohesive. Set aside.