Cream the softened butter with the powdered sugar until smooth and fluffy. Gradually mix in the flour until the dough just comes together. Do not overmix.
Roll the dough between two sheets of parchment paper until it’s about 1/2 inch thick. Remove the top sheet, leaving the dough on the bottom sheet. Cut the dough in half crosswise, then lengthwise into quarters. Divide each quarter into four equal rectangles to make 16 fingers. Gently prick each one with a fork. Leave the dough on the bottom parchment sheet and slide it onto a rimless baking sheet. Chill for 15 minutes.
Preheat the oven to 325°F (160°C). After chilling, carefully separate the cookies on the parchment paper so they are spaced slightly apart. Bake for 18-20 minutes, until the edges are lightly golden. Cool completely on a wire rack.
To Dip in Chocolate
Melt the chocolate with the oil together in the microwave at 50% power, stirring every 30 seconds, or in a heatproof bowl placed over simmering water. Stop heating when the chocolate is about 80% melted and stir vigorously until smooth.
Dip one end of each cooled shortbread finger into the melted chocolate, letting any excess drip off. Place on parchment paper and let the chocolate set at room temperature for at least 1 hour before storing.
Notes
Unsalted Butter: If using unsalted butter, add 1/4 tsp sea salt of salt to balance the flavors. Chill the dough before baking to help the fingers hold their shape.Cookie Cutters: You can roll the dough out and cut shapes with cookie cutters, if you prefer.Storage: Store cookies in an airtight container or freeze for up to 3 months.