Preheat the oven to 180 °C (356°F).
Create a double boiler using a medium-sized saucepan and a glass bowl. Place the glass bowl on top of the water-filled saucepan and set it over low heat. Make sure that the bottom of the glass bowl does not touch the water. Add the dark chocolate to the glass bowl and allow it to melt (1-2 minutes). Once the chocolate is melted, remove the saucepan from the heat. Then add the brown sugar, granulated white sugar, vegetable oil to the melted chocolate and whisk until combined. Now, Allow the mixture to cool down to room temperature.
Next, add the eggs and vanilla extract to the melted chocolate and sugar mixture and whisk until combined.
In a separate bowl, whisk the flour, xanthan gum, salt, and cocoa powder. Then add the melted chocolate and sugar mixture to the flour mixture and mix using a spatula.
Align a 20 x 20 cm ( 8 x 8 inch) square pan with parchment paper. Then pour half of the brownies batter. Next, evenly place 10 pieces of kinder chocolate on the first brownie layer. Afterward, cover the kinder chocolate with the remaining batter. You can add pieces of kinder chocolate to the top of the brownies if you like.
Bake the brownies for 27-30 minutes or until the edges are firm.