• Preheat the oven to 350°F and line two baking sheets with parchment paper and set aside.
• Mix all dry ingredients and set aside.
  1¼ cup All-purpose flour, ¾ teaspoon Baking powder, ¼ teaspoon Salt
• Brown the butter by cooking the butter on medium heat until it is a golden color and small brown specks form. Set it aside to cool down and then add the water.
  ½ cup Butter, 2 tablespoon Water
• Beat the brown butter with both sugars until well-mixed and fluffy.
  ¼ cup Granulated sugar, ¾ cup Brown sugar
• Add the egg, yolk, and vanilla extract, and mix until eggs are well combined.
  1 Egg, 1 Egg yolk, 1 tablespoon Vanilla extract
• Gently mix in the dry ingredients and form the dough. Do not overmix.

Marbleing

• Divide the dough into two equal parts. Add the ¼ cup flour to one portion and ¼ cup cocoa powder to the other half and mix.
  ¼ cup All-purpose flour, ¼ cup Cocoa powder
• Take a small amount of each dough and press them together and make a disk. Tear this disk into two parts and flip one part and attach them again. Tear this disk again and repeat the above step one more time. Roll the dough in your hands. If the dough is sticking to your hands, wet your hands with a little bit of warm water.
• Place the balls on the baking sheets, and bake for 13-15 minutes. They are done once the edges are set.
• Let them cool down on the sheets for 10 minutes. then transfer them to the cooling rack.