How to Make It
In a bowl, beat the butter with the sugar at medium speed until fluffy. Fold in both flours, the almond extract and half of the beaten egg; reserve the rest of the egg for another use. Cover the bowl with plastic wrap and refrigerate the pastry for 30 minutes.
Preheat the oven to 350°. Divide the pastry into 18 equal pieces. Press each piece evenly into the bottom and up the sides of a 1-ounce tartlet mold (our molds were 4-by-1 3/4-by-1/3 inches). Bake for 18 to 20 minutes, until the pastry is golden and set. Transfer to a rack to cool completely. Remove the tartlet shells from the molds.
In a saucepan, combine two-thirds of the chopped plums with the sugar and 2 tablespoons of water; bring to a boil. Simmer over moderately low heat, stirring occasionally, until very thick, about 30 minutes. Stir in the lemon juice and cook until slightly reduced,
5 minutes. Transfer to a blender and puree until smooth. Scrape the puree into a bowl and let cool; cover and refrigerate until firm and cold.
In a small saucepan, combine the cream with the remaining plums and all of the crushed plum pits. Bring just to a simmer and cook over low heat until the cream is reduced to 1 1/4 cups, about 30 minutes. Strain the cream into a medium bowl and let cool to room temperature, about 30 minutes. Cover and refrigerate until chilled, 1 hour.
Beat the cream to soft peaks. Spoon about 1 tablespoon of the plum puree into each tartlet. Top with a dollop of the plum cream and serve.