1. Preheat the oven to 375ºF (190ºC.) Butter a 13-inch rectangular tart pan or another pan (see headnote.)
2. In the bowl of a stand mixer, or by hand in a large bowl, make the cookie base by creaming the butter with the sugar until very light and fluffy, about 5 minutes. In a separate bowl, sift together the flour, baking powder, and 1 1/2 teaspoons ginger.
3. Mix the dry ingredients into the creamed butter mixture until well-combined. Turn the dough out onto a lightly floured countertop and knead it until the dough is relatively smooth.
(If necessary, dampen your hands to add a bit of moisture to the dough, if it’s dry.)
4. Press the dough into the prepared pan and flatted the surface, then bake the dough for 20 minutes, until it’s light golden brown.
5. Five minutes before the dough is done, making the icing by heating the 2 1/2 ounces of butter and golden syrup in a small pan, then mix in the powdered sugar and 1 tablespoon ginger, stirring until smooth.
6. When you take the pan out of the oven, pour the warm icing over the cookie base. Let sit for about 20 to 30 minutes, then remove from the pan and slice while still slightly warm.