Melt the 70g of butter and crush the digestive biscuits. Mix them together and press into a lined and greased 22cm round springform pan and place in the fridge while you make the brownie mixture.
Preheat the oven to 160°C (325°F).
In a medium sized saucepan, melt the butter and condensed milk together. Once melted remove from the heat and stir in the brown sugar and vanilla extract.
Stir through the cocoa, flour and baking powder. Pour into the prepared baking pan over the biscuit base.
Bake for around 35-45 minutes or until the sides are set and the middle has the slightest wobble.
Serve warm or place in the fridge to become ultra chewy!