The Chocolate Chip Cookies of my dreams!

Bahan-bahan (8 porsi)

• 225 g / 2 US sticks unsalted butter (, cut into 1cm / 1/2" cubes)
• 2 cups (300g) plain flour / all-purpose flour (, spooned and levelled (IMPORTANT - Note 1))
• 3 tsp cornflour/cornstarch (, tightly pack and level the spoon measure)
• 1/4 tsp baking powder
• 1/2 tsp baking soda / bi-carb (, sifted if lumpy (don't sub more baking powder, Note 2))
• 1/2 tsp cooking salt / kosher salt ((halve for table salt, +50% for flakes))
• 1 tightly packed cup (200g) brown sugar ((light brown sugar))
• 1/2 cup (100g) caster sugar / superfine super ((sub regular/granulated))
• 1 large egg ((55g / 2 oz in shell), at room temp)
• 1 yolk from a large egg (, at room temp)
• 1 tsp vanilla extract
• 1 1/2 cups (250g) dark chocolate chips (/ semi-sweet choc chips, plus extra for decorating)
• 3/4 cup (150g) milk chocolate chips