To Bake the Cake
Set the rack in the middle level inside the oven and then preheat the oven to 180°C. Line a cake pan with baking paper.
Boil 2 tbsp of water and add 2 tbsp of hazelnut flavoured instant coffee over medium-low heat. Stir until concentrated coffee is formed.
Add dark chocolate, vegetable oil, and unsalted butter and stir well until the mixture is smooth and well combined. Remove the pot from the heat and reserve.
Sift cake flour, salt, baking soda, and baking powder into a large mixing bowl.
Add sugar into the mixing bowl and mix well using a whisk.
In another bowl, mix eggs, plain yoghurt and vanilla extract. Mix well using the whisk.
Combine the coffee-chocolate mixture and the flour mixture together. Stir well using a spatula until the flour is just combined.
Add the egg mixture and continue mixing using the spatula and/or whisk until the batter is smooth.
Pour the batter into the prepared baking pan and bake for 18 mins.
Test with a toothpick to check if the mocha cake is cooked. If the flour is still not done, bake for a further 5 mins more. If the flour is well cooked, leave it to cool down while making the toffee and nut topping.
To Make the Toffee and Nut Topping
Sift caster sugar, salt, and flour into a mixing bowl. Set aside.
Add evaporated milk and hazelnut coffee into a saucepan and bring to a boil over low heat. Stir until they are well mixed.
Add butter. Continually stir until the butter is melted.
After the butter is completely melted, increase the heat to medium-low, add the reserved flour mixture into the pot and continually stir while adding the mixture.
Remove the pot from heat when the topping boils and then add the chopped nuts.
Pour the nuts topping over the mocha cake. Using the spatula to spread the topping all over the cake evenly.
Switch oven to grill setting at 180°C (do not place the cake too close to the element) and grill for 10 minutes until the topping caramelises.
Remove from the oven and allow the toffee cake to cool down.
Transfer to a wire rack to cool completely.
Use a sharp knife to cut the cake. Serve.