Sift all-purpose flour and unsweetened cocoa powder into mixing bowl. Set aside.
Add unsalted butter and confectioners’ sugar to stand mixer. Beat on low 1 to 2 minutes or until well combined, smooth and creamy.
Add egg yok and vanilla bean paste. Beat on low for 30 seconds. Scrape down sides of bowl, if necessary.
Add sifted flour and cocoa powder into butter mixture. Beat until the dough comes together, about 30 seconds to 1 minute.
Remove the dough from stand mixer bowl. Shape into a disc and wrap in plastic wrap. Chill at least 1 hour.
Remove dough from fridge and warm up for several minutes.
Place dough on plastic or wax paper. Press the dough down with your hands first to expand the disc size and flatten. Using rolling pin, roll the dough into an 11-inch circle. Transfer dough to 9-inch tart pan with removable bottom. Press dough into sides and bottom of tart pan. Chill for 30 minutes.
While the dough is chilling, preheat the oven to 325 degrees F.
Remove the tart from the fridge. Prick the bottom with a fork about a dozen times. Line the tart dough with parchment paper and fill with pie weights, dried beans or dried rice. Bake 15 minutes. Remove parchment and pie weights. Bake 5 additional minutes. Remove from oven and cool on wire rack.