1cup(2 sticks) unsalted butter,softened to room temperature
2largeegg yolks,at room temperature
1teaspoonpure vanilla extract
For the chocolate ganache filling:
4ouncessemi-sweet Baker's or couverture chocolate,chopped (about ½ a cup)
¼cupheavy whipping cream
Make the chocolate cookies: In a medium bowl, whisk together flour, cocoa powder, baking powder and salt. Set aside.
In a large bowl, combine butter and sugar. Cream together using a wooden spoon or an electric mixer until the sugar is fully dissolved (about 1 to 2 minutes). Add your egg yolks and vanilla extract to the butter mixture, and whisk until combined.
Combine the dry ingredients and wet ingredients. Mix until you start to get a rough ball of dough.
Divide the dough into 2 disks, and wrap each disk in plastic wrap. Chill in the fridge for at least 1 hour before rolling and cutting.Preheat your oven to 350°F. Line 2 large baking sheets with parchment paper or a silicone baking mat.
Unwrap your first disk of chilled dough and give it a quick knead on a lightly floured work surface (just to warm it enough to roll easily). Alternatively, you can take the dough out of the fridge about 15 minutes before rolling to let it warm slightly. Roll the dough out into about a ¼ inch thick. Using a 2 inch round or fluted cookie cutter, cut out 24 plain cookies. You'll need to gather and reroll the dough a few times to get 24 cookies.Arrange the cookies onto your baking sheet, leaving a bit of room to allow air to circulate. The cookies won't expand too much, but they do need a tiny bit of room to breathe!
Repeat the rolling and cutting process with the other disk of dough, but this time, cut out a small center hole in each cookie (I just used a piping tip to do this, but you can also use a small cookie cutter or bottle cap). Arrange the cookies with a center cutout on your other baking sheet.
Bake the cookies in the center of your preheated 350°F oven for 12 to 15 minutes, or until no longer shiny on top and slightly puffed. The cookies with the center cutout tend to bake within 12 minutes, while the plain cookies without the cutout usually take about 14 to 15 minutes. Cool completely before assembling.
Make the chocolate ganache: In a microwave/heat safe bowl, combine your chopped chocolate and heavy cream. Heat in your microwave or over a bain marie until the chocolate is melted, and you're left with a smooth mixture.If melting in your microwave, pop the bowl into your microwave and heat on high for 30 seconds. Take the bowl out and give it a mix. Pop the bowl back in and microwave on 10 second intervals, pausing after each one to stir, until the mixture is melted and smooth.If melting over a bain marie, bring a pot filled with 1 inch of water to just under a simmer. Place the bowl of chocolate/cream over the pot (making sure the bottom of the bowl does not touch the water). Heat, stirring occasionally, until melted and smooth. Cool the chocolate ganache to room temperature to allow it to firm up slightly before filling the cookies.
Dollop just under ½ a teaspoon of ganache onto the tops of the cookies without a center cutout. Sandwich each one with a cookie that has a center cutout.
* For a darker and richer color to your cookies, you can use Dutch process cocoa powder instead.