Place peach slices into a large bowl, and sprinkle with brown sugar. Stir to coat, and set aside.
In a medium metal bowl, stir together the white sugar, flour, lemon zest and salt.
Whisk in the lemon juice until the mixture is smooth. Gradually whisk in the boiling water.
In a smaller bowl, whisk the yolks together. Gradually whisk in about 1/2 cup of the hot lemon mixture. Then whisk the yolk mixture back into the larger bowl. Set the bowl over a pan of simmering water. Cook, stirring frequently until the mixture is thick enough to coat the back of a metal spoon.
Remove from the heat and whisk in the butter. Set aside.
In each parfait glass, make a layer of pound cake cubes, about 1/2 cup. Top with about 2 tablespoons of lemon sauce, then 1/4 cup of peach slices. Repeat the layers one more time. Top with 1 tablespoon of the lemon sauce. Refrigerate for at least 1 hour to be sure it is thoroughly chilled. Top each parfait with a dollop of whipped cream before serving.
