How to Make It
Heat oven to 350° F. Whisk together the flour, cocoa, and salt in a bowl.
Neat the butter, sugar, and vanilla in a bowl with an electric mixer on medium-high until light and fluffy, 2 to 3 minutes. Reduce mixer speed to low and gradually add the flour mixture; stir until just combined.
Press the dough into a jelly roll pan or a rimmed baking sheet (which makes slightly thinner shortbread). Cut the dough into 50 1-by-3-inch pieces, then prick all over with a fork. Bake, rotating the pan halfway through, until just starting to pull away from the edges of the pan, 26 to 28 minutes. Let cool for 5 minutes, then recut along the scored lines. Let cool completely in the pan.
Combine the cream and chocolate in a microwave-safe bowl. Microwave on high in 30-second intervals, whisking in between, until melted and smooth. Let cool slightly. Dip one end of each shortbread finger in the chocolate. Transfer to a wire rack and allow to set.