3 sticks unsalted butter, at room temperature, plus more for greasing
1/2 cup Hand-Cut Orange-and-Lemon Marmalade
Vanilla ice cream and mint leaves, for serving
Preheat the oven to 300°. In a large bowl, mix the flour and sugar. Add the butter and gently work it into the flour with your fingers until a dough forms.
Line a 10-by-15-inch rimmed baking sheet with parchment paper; butter the paper. Press the dough evenly into the prepared pan, about 1/3 inch thick. With a knife, score the dough into 2 1/2-by-2-inch bars. Bake the shortbread for about 50 minutes, until pale golden and set. While the shortbread is still hot, cut it into bars. Transfer the baking sheet to a rack and let cool.
Set 1 shortbread bar on each plate and spoon about 1 tablespoon of the marmalade on each one. Top each shortbread with ice cream, garnish with mint and serve.
The shortbread can be kept in an airtight container for up to 3 days or frozen for up to 1 month.