1. Preheat your oven to 350°F (175°C). Butter and line a 9-inch (23 cm) cake pan.
2. Combine the butter, granulated sugar, brown sugar, chocolate, and golden syrup in a saucepan. Cook over low heat, stirring constantly, until melted and smooth.
3. Remove the saucepan from the heat.
4. In a large bowl, whisk the eggs until light and fluffy. Stir in the vanilla extract, flour, baking powder, cocoa powder, and the melted chocolate mixture.
5. Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
6. Let the cake cool in the pan for 20-30 minutes before turning it out onto a wire rack to cool completely.
7. Serve the cake warm or at room temperature with your favorite toppings.