Chocolate & hazelnut slice

Ingredients

• Chocolate and hazelnut slice
• 1 1/2 cup hazelnuts, roughly chopped, some left whole
• Base
• 1 1/2 cup plain flour (gluten-free flour works well too)
• 1 teaspoon baking powder
• 4 tablespoon cocoa
• 1/2 cup brown sugar, loosely packed
• 1 egg
• 100 gram butter, melted and cooled slightly
• Topping
• 1 1/2 cup brown sugar
• 100 gram butter
• 1 teaspoon vanilla extract
• 1/4 cup water