The filling is similar to my favorite lemon curd, except for 2 adjustments. First, there’s the addition of heavy cream, which is optional but adds richness and great taste, and you can use up to ¼ cup of it.
Second is the amount of butter. I usually don’t like a lot of butter in my lemon curd, but here it helps to mute the bold lemon flavor, which I find really makes this tart superb. You can cut the butter by half if you prefer, or on the contrary, add more. The butter also adds a thick, creamy texture and the long whisking when adding it makes it light and airy. Instead of whisking the butter by hand, you can use a blender; just add the lemon mixture and blend, adding a few cubes of butter at a time, until completely smooth.
Chill the tart for several hours before serving or the curd will be too runny. After enough chilling time, the lemon curd will firm up, making it easier to cut through the pie.
An interesting and delicious option is to add basil to the lemon curd. Add ½ cup packed fresh basil leaves along with the sugar, eggs, and other ingredients, and let them cook together. You’ll discard the leaves upon sieving the mixture through a strainer.
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1 fully baked tart crust
For the lemon curd:
2 large eggs, plus 2 egg yolks (or 3 whole eggs)
¾ cup (150 g/5.3 oz) granulated sugar
1 tablespoon lemon zest
½ cup (120 ml) freshly squeezed lemon juice (2-3 lemons for both zest and juice)
2 tablespoons heavy cream, optional
½ cup (1 stick/115g) unsalted butter, cut into small pieces