Preheat the oven to 350°F (180°C) or 160°C fan oven, line 3 baking sheets with parchment paper or silicone baking mats and set aside.

In a large bowl, whisk together the flour, cooca powder, baking soda and salt; set aside

In a separate bowl, cream together the butter, brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.

Add the flour mixture to the butter mixture and stir until just combined.

Divide the dough using a medium cookie scoop into 20 equal sized dough balls ( I used the medium OXO cookie scoop, 1.5 tbsp size and each ball was about 24-25 grams). Gently push a rolo into the center of each dough ball. Pinch the sides of the dough ball until you can pinch the dough over the top of the rolo, sealing the rolo inside the cookie dough.

Roll each dough ball in granulated sugar before placing on the prepared baking sheet 2 inches apart.

Bake in the preheated oven for 9 minutes, or until the cookies have spread and are slightly puffed. Allow to cool for 5 minutes then transfer to a wire rack to cool completely.