Preheat the oven to 160°C (140°C fan forced) and line 2 large baking trays with baking paper. Melt the chocolate and butter in a small saucepan over low heat until smooth. Set aside to cool slightly.
Combine the flour, brown sugar, caster sugar, cocoa powder and bicarbonate of soda in a large bowl. Then add the egg and vanilla followed by the chocolate mixture until combined.
Roll approximately 1 tablespoon portions of cookie mixture into balls. Place 5cm apart on the prepared trays and lightly flatten with two fingers before baking for 18 mins. Set aside on a wire rack to cool. Meanwhile line the base of 22cm springform cake tin with baking paper.
Process the full batch of biscuits until crumbs form. Add 100g butter, melted and stir until combined. Press crumbs into the base of a 22cm springform cake tin and refrigerate for 45 minutes until set.
Process the cream cheese, sour cream and caster sugar using the stand mixer fitted with the whisk attachment on medium speed. Beat in 3 eggs until mixture is smooth.
Transfer 1⅓ cups of cheesecake mixture into a medium size bowl. Add 100g dark chocolate, melted and stir until combined. Pour half of plain mixture onto the biscuit base. Then dollop with half chocolate mixture. Repeat. Use skewer or knife to swirl or simply mix.
Place the cheesecake on a baking tray and bake for 1 hour. Turn the oven off and leave the cheesecake to cool slightly in the oven for 20 minutes, with the door ajar. Then transfer to a wire rack and cool completely in refrigerator until ready to serve.