These shortbread squares are versatile — one cookie, three delicious variations.
To make Chocolate-Peppermint Shortbread Squares: Using electric mixer, beat butter on medium speed until smooth. On low, gradually add confectioners’ sugar until fully incorporated. Add heavy cream and pure peppermint extract and beat 2 minutes. Spread over cooled crust and refrigerate until firm, at least 30 minutes. In medium bowl, melt chocolate chips and butter (cut into pieces) in microwave on 50% power in 30-second intervals, stirring after each interval until melted and smooth. Spread onto peppermint layer and refrigerate until set, at least 30 minutes. Cut into pieces.
To make Toffee-Pecan Shortbread Squares: In heavy-bottomed medium saucepan, combine sugar and water. Heat on medium (do not stir), swirling pan occasionally, until sugar has dissolved. Increase heat and boil until sugar is deep caramel color (do not stir). Immediately remove from heat and stir in heavy cream (it will bubble up), then fold in toasted pecans (very roughly chopped). Pour mixture over cooled crust and sprinkle with flaky salt. Let cool completely, then cut into pieces.
To make Red Currant Crumble: Prepare Shortbread Dough, but transfer 3/4 cup dough to piece of plastic wrap, roll into log and freeze until firm, 30 minutes. Spread remaining dough into pan and bake 12 minutes; let cool. In bowl, combine jam with orange liqueur and orange zest and spread over dough. Using box grater, coarsely grate frozen dough over top. Bake until topping is golden brown, 35 to 40 minutes. Let cool, then cut into pieces.