Preheat oven to 325F. Line a 9x9-inch brownie pan with foil (or spray the bottom and sides with floured baking spray, if preferred). You can also use an 8x8-inch brownie pan, which will yield a thicker cake.

In a large bowl, combine the butter, brown sugar, corn syrup, vanilla extract and eggs. Mix well with a rubber spatula until you have a thick, cohesive mixture.

In another bowl, whisk together the flour, baking powder and salt. Add the chopped dates and mix with your hands, separating pieces that are stick together, and making sure all the pieces are dredged with flour. Add the walnuts and dredge them in the flour as well.

Fold this dry mixture into the wet ingredients using the spatula, until you can no longer see any flour remaining. You will have a very thick batter. Transfer the batter to the prepared brownie pan, evening out the surface as much as possible.

Bake for 40-45 minutes, turning the pan around halfway through the baking, until the top is nicely browned and looks crusty, and a toothpick inserted in the middle of the cake comes out clean. Transfer the baking pan to a rack to cool completely before slicing and serving.

The cake squares will keep, tightly wrapped, at room temperature for a couple of days, or for about a week in the refrigerator. Gently reheat in the microwave for about 10 seconds before eating.