Preheat and prep. Heat your oven to 325°F (160°C). Place 4–6 small ramekins in a deep baking dish. Boil a kettle of water for the water bath.

Warm the cream. In a small saucepan, gently heat 1 1/2 cups heavy cream over low heat until warm to the touch but not simmering. Remove from heat. If using, whisk in 1 tablespoon unsalted butter until melted.

Mix the lemon base. In a bowl, whisk 1/2 cup granulated erythritol or allulose with a pinch of salt, the finely grated zest of 1 lemon, and 3 large eggs until smooth. Whisk in 1 teaspoon vanilla extract and 2–3 tablespoons fresh lemon juice (adjust to taste).

Temper the eggs. Slowly stream the warm cream into the egg mixture while whisking constantly. This prevents scrambling and creates a silky texture.

Strain for smoothness. Pour the custard through a fine-mesh sieve into a large measuring cup or bowl to catch any zest clumps or egg bits. This step makes the custard extra smooth.

Fill the ramekins. Divide the custard evenly among the ramekins. Tap them gently to release any surface bubbles.

Make a water bath. Slide the baking dish into the oven. Carefully pour hot water into the dish until it reaches halfway up the sides of the ramekins.

Bake until just set. Bake 25–35 minutes, depending on ramekin size. The centers should jiggle slightly when nudged but not slosh. Do not overbake or the custard may curdle.

Cool gradually. Remove the ramekins from the water bath and let them cool on a rack. Chill for at least 2 hours, or overnight for best flavor and texture.

Serve. Enjoy plain, or add a few fresh berries and a light sprinkle of lemon zest. A tiny dollop of whipped cream is a nice touch if it fits your macros.