Preheat oven to 350℉ and grease and line an 8x8-inch square baking pan with parchment paper.
In a large bowl, whisk together the ½ cup unsalted butter, ½ cup pure pumpkin puree, ½ cup light or dark brown sugar, ⅓ cup granulated sugar, 1 large egg, and 1 teaspoon pure vanilla extract until smooth, then fold in the 1 ½ cups oat flour, 1 teaspoon baking powder, ½ teaspoon ground cinnamon, and ½ teaspoon fine sea salt. Stir in most of the ½ cup semisweet chocolate chips and ½ cup chopped walnuts, reserving some to scatter over the top.
Pour the batter into the prepared pan and smooth the top, then sprinkle it evenly with the remaining chocolate and nuts. Rest the batter for 15 minutes, then bake for 35-40 minutes, until set and a tester comes out clean.
Place the pan on a rack to cool for at least 20 minutes, then cut into 16 pieces.
Brown the butter. For an even deeper, more deliciously complex and buttery note, brown the butter first. Place the butter in a pan over medium heat and melt. Stir and swirl the pan until it foams, turns golden brown, and has a slight nutty aroma. Remove from the heat and cool slightly before proceeding with the recipe.
Cool before cutting. Just like with brownies, you want the blondies to fully cool before you slice them. This makes for neater, cleaner cuts.
Double the batch. For a bigger batch, double the recipe and bake in a 9x13-inch baking pan.
Use a chocolate bar. Full disclosure - I tend to reach for the bag of chocolate chips because it cuts down on time. But really, it doesn't take much extra to chop up a chocolate bar. Chopping the chocolate ensures that the chocolate melts into the batter, adding a smoother texture and more luxurious touch.
Try different chocolate + nuts. I mentioned this above, but rather than a straight swap, you could also do a mix of different chopped nuts and different chocolate varieties.
Dairy-free. Use melted non-dairy butter or margarine, or melted coconut oil.
Vegan. I haven't made these blondies with an egg substitute, but I imagine a flax egg would work very well since pumpkin already works to create natural structure.
Naturally sweetened option. Instead of the brown and granulated sugars, use coconut sugar or a mix of coconut sugar and maple syrup.