Preheat the oven to 350°F. Grease a 9x9 inch baking pan or line it with parchment paper.
In a large mixing bow whisk together the brown sugar and butter.
Add in the vanilla, pumpkin puree, and egg yolk, and whisk until well combined.
Then add in the flour, salt, baking powder, and pumpkin pie spice. Stir until fully incorporated.
Add the white chocolate chips, and fold into the batter.
Spread into a greased (or parchment lined) 9x9 inch baking.
Bake in a preheated oven for 20 to 25 minutes, until the edges are set, but the center is still a little soft. (Cook longer for a more cake-like texture).
Remove from the oven and let cool before slicing and serving.