Cream the softened butter on medium speed until light and fluffy, about two minutes. Add the sugar and egg yolks and beat until completely incorporated.

Mix together the flour, baking powder, and salt. With the mixer on low, add the dry ingredients to the butter until just incorporated.

Divide the dough in half and shape each into a narrow log. Wrap each log in plastic wrap and freeze for at least 30 minutes (and hour is better). Remove from freezer.

Preheat oven to 350°. Butter one of the following: an 8" springform pan, or a 7x7" square cake ring. If using a cake ring, line a small baking sheet with parchment paper and place the ring on top.

Grate the dough on the large holes of a box grater into a bowl. Alternately (and much faster), grate the dough in your food processor, using the grating disk. If you break the logs in half or thirds, they can fit into the shoot on their sides for fast grating.

Transfer the dough to a large mixing bowl. Add the chips into the dough grates and gently toss. Pour into the prepared pan or ring, and distribute evenly, pressing lightly to the edges.

Bake for 20-25 minutes, or until the top is lightly golden. Remove and cool in the pan, on a wire rack.

Heat in the butter in a small saucepan over medium until melted and beginning to sizzle. Add the Kahlua and vanilla heat briefly. Place the chocolate and cocoa powder in a bowl, and pour the hot butter mixture over. Stir until the chocolate is fully melted, then let the glaze set for 10 minutes.

Drizzle glaze attractively over the cooled shortbread, or pour over the top and even out with a spatula. Let set - or not! - and serve.