This berry filling is tart and fruity, and superbly balanced by the crisp, buttery shortbread crust.
In a bowl, using an electric mixer, beat the 1 stick plus 2 tablespoons of butter with the sugar and egg yolk until smooth. Add the flour, baking powder and salt and beat at low speed until evenly moistened, about 2 minutes. Using your hands, knead once or twice to bring the dough together.
Butter a 9-inch square metal baking pan. Pat three-fourths of the dough into the baking pan in an even layer. Roll the remaining dough into 36 small balls. Roll the balls in sugar, transfer to a wax paper–lined baking sheet and press the balls to 1/4-inch-thick rounds with the bottom of a glass. Refrigerate the crust and rounds for 10 minutes. Preheat the oven to 350°.
In a microwave-safe bowl, cover the cranberries with water and microwave at high power for 1 minute, just until plump. Drain, pressing out the water; let cool slightly. Return the cranberries to the bowl and stir in the preserves and lemon zest.
Spread the berry filling over the dough in the baking pan and arrange the rounds on top. Bake in the lower third of the oven for 40 to 45 minutes, or until the top is golden. Let the square cool completely.
Run the tip of a knife around the edge of the baking pan. Cut the square into 18 bars and serve.