Whisk together the sugar, corn syrup, water, and lemon juice in a microwave-safe measuring cup or glass bowl until combined. Microwave on high for 5 to 8 minutes, until the mixture ever so slightly begins to turn a light golden color. Watch carefully—if you wait even a few seconds too long, it will turn too dark, and will thus overcook and turn slightly bitter.

Carefully remove the measuring cup from the microwave and place it on a dry surface. Let it rest untouched for about 3-5 minutes, or until the mixture turns a deep amber color.

Then add in about a tablespoon of the heated cream, and immediately whisk to combine. The caramel will bubble furiously, but whisking will keep it from overflowing. Continue adding the cream slowly in small increments until it is all whisked in. Then add in the vanilla and butter and whisk to combine. Serve immediately warm, or let it sit and come to room temperature to thicken slightly.

Store in a jar or sealed container and refrigerate for up to 2 weeks.

Heat your cream either in the microwave or a (separate) small saucepan until warm but not boiling.