For the Cookies

1. In a large bowl with a hand mixer or in the bowl of a stand mixer, cream the butter and sugar together until smooth on medium speed. Add vanilla extract, egg, and egg yolk one at a time. Beat until fully combined.
2. In a medium bowl, whisk flour, powdered sugar, baking powder, and salt.
3. Slowly add dry ingredients to the wet ingredients, mixing on low and making sure not to overmix. Batter will be soft and sticky.
4. Divide the batter in half, smoothing each half onto a piece of cling wrap to form a disk of dough. Thoroughly wrap each dough disk in more cling wrap before placing them in the fridge for one hour.
5. Preheat your 350 degrees Fahrenheit. Line several baking sheets with parchment paper. You’ll be baking about 40 disks of dough in total, and they need to have room to between them on the sheets.
6. On a clean, lightly floured surface, roll out one dough disk to be about 1/4 inch thick using a rolling pin. You do not want to go any thicker than this unless you are planning to increase the amount of filling you make. The cookie will be too dry.
7. Using a 2.5 inch round cup, cookie cutter, or pastry cutter, cut as many circles as you can from the dough, transferring them to a parchment-lined baking sheet. You will need to form the leftover dough into a ball and use a rolling pin to reroll the dough into a 1/4 inch thick slab.
8. Repeat the process with the second dough disk, rolling to 1/4 inch thickness and cutting into 2.5 inch rounds.
9. With these dough disks, use a 1 inch round cookie cutter, piping tip, or core tool to create a hole in the center of these rounds. By the end, you should have about 20 full rounds and 20 donut rounds.
10. Make sure your donut rounds are on separate baking sheets from your regular rounds, as the donut rounds cook for about 1-2 minutes less.
11. Bake the full rounds at 350 degrees Fahrenheit for 10-12 minutes. The edges should just be starting to turn a light gold in color, but it might be difficult to see. The tops of the cookies will be light in color and look dry.
12. Bake the donut rounds at 350 degrees Fahrenheit for 8-10 minutes. Just as the full rounds, the edges should just be starting to turn in color, but the tops will be light, puffy, and dry looking.
13. Allow the cookies to fully cool before assembling.

For the Ganache Filling

1. Place semisweet chocolate chips or chopped semisweet bar into a medium bowl.
2. In a medium saucepan over medium heat, heat the heavy whipping cream until it JUST starts to simmer. It should NOT boil. You are simply looking to heat it enough so that it will fully melt the chocolate chips.
3. Remove the heavy whipping cream from the heat, and pour it over the chocolate. Ensure that all of the chocolate is either covered by the cream or has been touched by the cream.
4. Cover the bowl with cling wrap for five minutes.
5. Using a whisk, start whisking in small circles in the middle of the bowl. Be patient, as the cream and chocolate will slowly come together. Begin moving your circle outwards, continuing to whisk towards the edge of the bowl. The ganache will be thick but slightly runny.
6. Allow the ganache to sit at room temperature for 30 minutes. This will help to thicken it.

For Assembly 

1. With a full disk (round cookie with no hole), spread about 1-1.5 tablespoons of ganache using a knife or spoon.
2. Place a donut cookie (round cookie with hole) on top of the ganache. Place to the side.
3. Repeat this process with remaining cookies.
4. Melt remaining chocolate chips in 30 second increments in the microwave. Repeat until the chocolate is smooth.
5. Transfer chocolate to a piping bag or Ziploc bag, cutting the very tip off of the bag to allow the chocolate to come out.
6. Run your hand back and forth over each cookie, applying pressure to the bag to release the chocolate. This will create the stripe.
7. Allow the cookies to set for at least an hour before serving. The ganache in the center will thicken and set over time, making the shortbread cookies softer. Store in an airtight container at room temperature for up to 3 days or in the fridge for up to 1 week.