Biscoff stuffed cookies

Bahan-bahan (8 porsi)

• 280 g / 9.5 oz biscoff spread (, smooth (8 x 1 tablespoon generously heaped, 35g/1 1/4 oz each) (Note 1))
• 225 g / 2 US sticks unsalted butter (, cut into 1cm / 1/2" cubes (Note 1))
• 2 1/4 cups plain flour / all-purpose flour (, spooned and levelled (IMPORTANT - Note 2))
• 3 tsp cornflour/cornstarch (, tightly pack and level the spoon measure (Note 2))
• 1/4 tsp baking powder
• 1/2 tsp baking soda / bi-carb (, sifted if lumpy (don't sub more baking powder, Note 3))
• 1/2 tsp cooking salt / kosher salt ((halve for table salt, +50% for flakes))
• 1 tsp cinnamon powder
• 1/2 tsp all spice powder ((sub mixed spice))
• 1/4 tsp ginger powder
• 3/4 cup (tightly packed) brown sugar ((light brown sugar))
• 1/3 cup 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 1/4 cups white chocolate chips (, plus extra for decorating)