Preheat your oven to 350°F (175°C). Line an 8x8-inch (20cm) square baking pan with baking paper, leaving extra overhang on all sides. This will make it easier to lift the bars out later.
Make the cookie dough. In a large bowl, use a hand whisk or electric whisk to cream the butter for 1–2 minutes until it becomes pale and fluffy.
Beat in the cane (castor) sugar and light brown sugar on medium speed for 3–4 minutes until the mixture is light and creamy. Scrape down the sides with a spatula.
In a small cup or bowl, combine the cocoa powder, vanilla and milk to form a smooth paste.
Add the cocoa-milk mixture to the butter-sugar mixture and mix until smooth and well combined.
Stir in the flour, baking powder, and salt until a soft dough forms.
Press the dough evenly into your prepared baking pan. Smooth the surface with a spatula or the back of a spoon.
Place in the preheated oven and bake for 25–30 minutes. The edges should be set but still slightly soft in the center.
Let the cookie base cool completely in the pan, and gently flatten any edges that may have risen with your finger tips .Make the chocolate ganache.