1. Line a 9- by 13-inch cake pan with foil, leaving an overhang on all four sides. (I overturn the pan, shape the foil over the bottom, remove it, then flip the pan over and ease the foil into the pan.) Spray the foil in the pan with nonstick spray or brush with melted butter.
Preheat the oven to 350ºF (180ºC).
2. To make the cookie base, whisk together the 1 1/2 cups (190g) flour, baking powder, and 1/4 teaspoon salt in a small bowl. Beat the 8 tablespoons (115g) of butter and the 3/4 cup (150g) of sugar in the bowl of stand mixer with the paddle attachment, or by hand, until light and fluffy, 3 to 5 minutes.
3. Add the egg and the milk and mix in, stopping the mixer to scrape down the sides of the bowl. Stir in the dry ingredients, until fully incorporated. Put the mixture in the cake pan in dollops (it’s too thick to spread if you add it all in the same place), and spread it into an even layer with an offset spatula.
(You don’t need to clean the bowl; you can reuse it for the next step.)
4. To make the soft gooey layer, in a small bowl, whisk together the corn syrup or golden syrup with the milk or cream, and vanilla.
5. Beat the 12 tablespoons (170g) of butter with the 1 cup plus 2 tablespoons (225g) of sugar, and salt until light and fluffy. Beat in the egg, scraping down the sides of the bowl.
5. Add one-third of the 1 1/4 cup (155g) flour, then half of the milk/corn syrup mixture. Add another one-third of the flour, then the rest of the milk/corn syrup mixture. Then stir in the remaining flour. Dollop the batter over the unbaked cookie layer and spread evenly.
6. Mix together the 2 tablespoons of sugar and cinnamon, then sprinkle it evenly over the cake.
7. Bake the cake for 25 minutes, or until the cake feels slightly damp, but gently set in the center. Remove from oven and let cool on a wire rack. When cool, lift out the cake using the overhang of the foil, and cut the cake into 1-inch (3cm) squares.