Whisk 1/2 cup white sugar, melted butter and brown sugar together in a large bowl. Add pumpkin puree and vanilla essence; whisk until smooth.
Combine flour, 1 1/2 teaspoons cinnamon, mixed spice, salt, bicarb and baking powder in a separate bowl.
Mix flour mixture into the sugar-butter mixture; stir until just combined. Fold in chocolate chips gently. Cover dough with plastic wrap and chill for 30 minutes.
Preheat oven to 175 degrees C. Line a baking tray with baking paper.
Roll 1 1/2 tablespoons of chilled dough into a ball. Repeat with the remaining dough.
Combine the remaining 1/2 cup white sugar, 3/4 teaspoon cinnamon and cayenne pepper in a small bowl. Roll dough balls completely in the spicy cinnamon-sugar. Place on the prepared baking tray, at least 5cm apart, and slightly flatten the tops.
Bake in the preheated oven until edges are golden and cookies appear under-baked, about 9 minutes. Remove from oven and immediately place candy-coated chocolate pieces on top. Cool on the baking tray for 10 minutes. Transfer to a wire rack to cool completely.
The dough can remain in the refrigerator for 2 days if necessary.