Preheat the oven to 175°C (350F) and line two large baking sheets with parchment paper. Make sure to have all ingredients ready and oven preheated as these brownie cookies require quick preparation and baking for best results.
Sift and mix flour, cocoa powder, baking powder, corn flour, and salt in a mixing bowl and set aside.
Place granulated sugar, brown sugar, and eggs in a bowl of a stand mixer fitted with the paddle attachment. Mix on medium speed for at least 5 minutes. In the meantime, melt the chocolate and butter.
Place butter and chocolate bar (or chocolate chips) in a microwave-safe bowl and heat for 20 second increments, stirring well after each 20 second heating cycle, until chocolate and butter are melted and smooth. Allow to cool for 2 minutes before adding to batter.If using a chocolate bar, break it down into small equal-sized blocks before melting.
Add butter-chocolate mixture and vanilla extract to mixing bowl containing the mixed sugar and egg mixture and mix for another 2 minutes on medium speed.
Add dry ingredients and mix on low speed until just combined. Mixture should be thick and gooey, but not too runny. If mixture is too runny, add 2-4 tbsp. of sifted flour and combine.
Fold in crushed hazelnuts using a spatula.
This step must be done immediately after finishing the brownie cookie batter, as quickly as possible.Use a cookie dough scoop (or roll dough into approximately 1 oz. or 30g balls) and place on baking sheet with parchment paper, leaving a 2 inch space between each one. Sprinkle with more crushed hazelnuts as desired. You should have 14-16 cookies.
Bake in a preheated oven for 12-14 minutes until cookies have spread and tops are cracked and shiny. (If you leave the batter to stand for too long, cookie surface will not be shiny). Once baked, centers remain soft and fudgy.
Cool for 2 minutes and then transfer to a wire rack to cool completely.