Preheat the oven to 180°C/gas 4. Line a 12-cup muffin pan with paper liners.
In a small bowl, combine the walnuts, 175g of the chocolate chips and 1 tablespoon of the brownie mix. Toss until all of the ingredients are coated. Set aside.
In a large bowl, mix together the remaining brownie mix, eggs, vegetable oil and water. Stir for 20 seconds until blended. Fold in the walnuts and chocolate chips. Spoon the mixture into the prepared pan (cupcake liners will be full). Bake for 22 to 25 minutes until a cake tester inserted into the cupcakes comes out with moist fudgy crumbs. Cool the cupcakes in the pan for 10 minutes. Transfer the cupcakes to a wire rack and cool completely before icing, about 30 minutes.
In a large bowl, using an electric hand mixer, beat the cheeses together until light and fluffy, about 2 minutes. Gradually beat in the icing sugar until smooth and spreadable. Refrigerate the icing until ready to use.
Spread the icing on top of the cupcakes and garnish with the remaining mini chocolate chips. Refrigerate until ready to serve.
Serve at room temperature.