Preheat your oven to 325°F (160°C) and line two baking sheets with parchment paper.

In a large mixing bowl, combine all-purpose flour, quick cooking rolled oats, dark brown sugar, white sugar, and salt. Whisk until well-blended.

In a saucepan over medium heat, melt unsalted butter until golden brown. Stir in golden syrup and water until fully combined.

Add baking soda to the browned butter mixture, then pour into the dry ingredients. Mix until a dough forms.

Using a cookie scoop, portion out the dough into balls and arrange them on the baking sheets, leaving space for spreading.

Bake the cookies for 8 minutes or until lightly golden around the edges.

To prepare the custard filling, bring water to a simmer with masala chai and fresh ginger for about 5 minutes, then strain.

In a clean saucepan, bring the strained masala chai syrup to a gentle boil. Whisk egg yolks, then gradually add the hot syrup.

Stir in custard powder and spices, cooking over low heat until thickened.

Incorporate softened butter into the custard until light and fluffy.

Spread custard filling on one cookie and top with another to create a sandwich. Refrigerate the assembled cookies before serving.