I had never heard of "Tan Square" (or "Tan Slice") before I came across a recipe for it on Cloudy Kitchen, but it's a classic New Zealand recipe. The bar has two components: shortbread (which serves as both the base and the topping) and homemade caramel.
Making the crust/topping is simple. You cream room temperature butter with sugar until light and fluffy; and mix in flour, baking powder, kosher salt, and vanilla paste. You reserve some of the mixture to use for the topping, press the remainder into a parchment-lined 9-inch by 13-inch pan, and chill the crust while you make the caramel.
For the caramel, you cook a can of condensed milk with golden syrup and butter until it has thickened and taken on a golden color; remove the mixture from the heat; add vanilla and salt; and pour the caramel on top of the chilled crust. Then you sprinkle the reserved shortbread mixture over the caramel and bake. The recipe suggests an optional addition of flaky salt when you take the bars out of the oven, but it just completely slipped my mind and I forgot to add it.
The proportions of shortbread to caramel seemed just right. I do think the bars would have been even better with a little Maldon salt sprinkled on top -- but the salt included in the caramel itself prevented the bars from being too sweet. The name "tan square" suggests something very boring and mundane, but don't let the name fool you!
Recipe: "Tan Square" from Erin Clarkson of Cloudy Kitchen.