Have ready a fine-mesh strainer suspended over a 2 cup glass measure with a spout and a second 2 cup glass measure with a spout alongside (or use one glass or porcelain container with a 5 to 6 inch flat base).

In a 1 cup microwavable measure with a spout (or in a small saucepan [see Notes] over medium heat), scald the cream (heat it to the boiling point; small bubbles will form around the periphery). Remove it from the heat and keep it warm.

In a small saucepan, with a silicone spatula, stir together the sugar and lemon juice until all of the sugar is moistened. Heat over medium heat, stirring constantly, until near the boiling point.

Remove the pan from the heat. Stir in the heated cream until the mixture is uniform in consistency. Pour it at once into the strainer and press it through into the glass measure. Pour half of the posset mixture into the second 2 cup glass measure (or all of the mixture into the glass container). The depth of the lemon posset should be no more than ¾ inch, which is necessary for the posset to firm up to the right consistency.

Refrigerate, uncovered, for 3 to 4 hours before filling the shortcakes. The posset will have a smooth sheen across its surface and form a custardlike layer to about halfway down from its surface. Do not stir the custard because it will not thicken properly and the thicker top layer is needed to seal the cake and keep most of the creamier posset from soaking into it.