Place the egg whites and sugar in the heatproof bowl from an electric mixer. Combine, using a wire whisk; continue whisking over a pot containing about 2 inches of simmering water.
Whisk constantly until all the sugar is dissolved and the mixture feels hot to the touch. Transfer the bowl to mixer and whip, using the whisk attachment, on high speed until whites are glossy and stand in stiff peaks when the whisk is lifted, about 3 minutes.
Heat oven to 175 degrees F. Make a pattern to use as a guide for piping by drawing 24 circles on a 16 by 12-inch piece of parchment paper; use a pencil to trace around a 2-inch biscuit cutter, creating 4 rows of 6 circles. Line a baking sheet with the patterned paper. Place a second piece of 16 by 12-inch parchment paper on top of the patterned paper (you will be able to see the circles through the parchment and be able to use the patterned paper again).
Using a medium-sized pastry bag with a #11 plain tip, pipe 7 petals, starting at the edge of each circle and pulling the bag into the center. (Make sure all points meet in center so that lemon curd does not seep out.) Slip the pattern from under the piped cookies, and use on the second sheet of cookies. With a wet index finger, make a well in the center of each daisy, being careful not to break through to the paper. Bake until meringue is dry in the center, about 1 hour. Remove to a rack to cool completely.
With a pastry bag fitted with a #11 plain tip, pipe 1/4 teaspoon of lemon curd in the center of each daisy. Chill cookies to set curd, about 5 minutes.
Combine yolks, sugar, lemon zest, and lemon juice in a small saucepan. Whisk to combine. Set over medium heat, and stir constantly with a wooden spoon, making sure to stir sides and bottom of pan. Cook until mixture is thick enough to coat back of spoon, about 5 to 7 minutes.
Remove saucepan from heat. Add the butter, one piece at a time, stirring with the wooden spoon until consistency is smooth.
Transfer mixture to a medium bowl. Lay a piece of plastic wrap directly on the surface of the curd to avoid a skin from forming; wrap tightly. Let cool; refrigerate until firm and chilled. Store in airtight container for up to 2 days.
Yield: 1 1/2 cups
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