Dough - In a food processor, add the flour, sugar, salt, and chilled cubed butter. Pulse for 30 seconds until it resembles coarse bread crumb consistency. Combine the egg yolk with cold water and add it to the mixture. Pulse or combine for 30 seconds more.
Chill - Pour the mixture onto a work surface. Bring all the crumbs together and shape them into a ball. Divide the ball into two discs. Wrap the pastry in plastic wrap and chill for 20 to 30 minutes or until firm enough to roll.
Tart pans - Roll the dough between two sheets of parchment paper to about 1/8 inch thickness. Use the tart rings to cut out the base of the rings. Then use a ruler to cut strips and line the sides of the rings, making sure the dough sticks to the ring properly. Chill the shells in the refrigerator for 15 minutes.
Bake - Preheat the oven at 375°F/190 °C/ Gas Mark 5 for at least 10 minutes. Then dock the base of the tart pans with the tines of a fork. Bake in the hot oven for 15 to 18 minutes until lightly golden. Cool completely on a wire rack.
Microwave - In a microwave-safe medium bowl, heat the chopped chocolate, heavy cream, butter, and corn syrup until all the chocolate is melted and smooth. Add salt, coffee, and vanilla extract. Double boiler - To make a double boiler at home, place a saucepan with 2 inches of water on medium heat. Place a heat-proof bowl over the saucepan and add all the ganache ingredients to the top bowl. Heat gently with a whisk until everything is melted and smooth. Saucepan - Heat the heavy cream in a small saucepan over medium heat until it comes to an almost boil. Pour the hot cream over the chopped chocolate and stir until smooth. Add the rest of the ingredients and stir until smooth.
Fill tarts - Pour the room temperature chocolate ganache into a piping bag and fill each tart shell.
Chill & Garnish - Chill in the fridge for at least 2 to 4 hours or until set. Add a chocolate-coated nut to each tart and sprinkle with powdered sugar. Serve at room temperature.