To Make the Cacao Nib Pastry Dough:

Position a rack in the center of the oven and preheat the oven to 350°F. Spray the inside of each tartlet pan with nonstick baking spray and place the pans on a baking sheet.

Place the walnuts in a cake or pie pan and toast in the oven for 7 to 8 minutes, until light golden. Remove the pan from the oven and cool on a rack. Chop the walnuts finely.

Pulse together the flour, brown sugar, and salt in the work bowl of a food processor fitted with a steel blade.

Cut the butter into small pieces and add it to the flour mixture. Pulse until the butter is cut into very tiny pieces, about 30 seconds. The texture will be sandy with very tiny lumps throughout. Add the cacao nibs and chopped walnuts and pulse briefly to blend.

In a small bowl, use a fork to beat the egg yolk and vanilla together. With the food processor running, pour this mixture through the feed tube. Process the dough until the mixture wraps itself around the blade, about 1 minute.

Break off walnut-size pieces of the dough and press them into the tartlet pans. Pinch off any excess dough at the top edge of the pans. Place another tartlet pan on top of each tartlet to act as a weight while it bakes.

Bake the tartlet shells for 7 minutes. Remove the top tartlet pans. If the bottoms of the pastry shells puff up, gently pierce them with a fork or the point of a knife to release the air. Bake another 7 to 8 minutes, until light golden and set. Remove the baking sheet from the oven and transfer it to a rack to cool completely. To remove the tartlet shells from their pans, gently tap each pan on the countertop.

To Make the Ganache Filling:

Place the chopped chocolate in a 2-quart bowl.

Place the cream in a medium saucepan and bring to a boil over medium heat. Pour the cream over the chocolate. Let it stand for 15 seconds, then stir together with a rubber spatula, whisk, or immersion blender until very smooth.

To Assemble:

Use a 1-inch round ice cream scoop to fill each tartlet shell to the top with ganache. Chill for 15 minutes to set the filling, or let it set at room temperature for 30 minutes to 1 hour.

Serve at room temperature.

Keeping:

Store the tartlets in a single layer on a baking sheet. Cover the tops of the tartlets with a large piece of waxed paper to keep the surface from becoming marred. Tightly wrap the pan with aluminum foil and keep in the refrigerator up to 3 days.

Streamlining:

The pastry dough can be made in advance and kept in the refrigerator, tightly wrapped in a double layer of plastic wrap, up to 4 days before using. To freeze up to 4 months, wrap it in a double layer of plastic wrap and enclose it in a freezer bag. Use a large piece of masking tape and an indelible marker to label and date the contents. If frozen, defrost the dough in the refrigerator overnight before using. If it is too cold to roll out, let it stand at room temperature to become pliable.

The tartlet shells can be baked and held at room temperature up to 2 days before filling. After they are completely cool, place them on a jelly-roll pan, cover with waxed paper, and tightly wrap the pan in aluminum foil.