Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper.
Put the oil, nondairy milk or mashed banana, and flaxseeds in a small saucepan. Heat until just steaming, then whisk in the brown sugar, rum (if using) and vanilla extract until smooth and well combined. Cool slightly.
Put the All-Purpose Flour Mix, baking powder, pumpkin pie spice (if using), xanthan gum, and salt in a large bowl. Whisk to combine.
Add the cooled oil mixture to the flour mixture. Mix to combine. Stir in the Chocolate Mega Chunks.
Scrape the mixture into the prepared pan. Bake for 22 to 25 minutes, until golden brown. Do not overbake.
Let the blondies cool completely in the pan before cutting into squares or cutting with a cookie cutter.
Serve, or store in an airtight container for up to 2 days, moving leftovers to the refrigerator or freezer.
Laurie Sadowski is the author of three cookbooks devoted to gluten-free, vegan, and allergy-friendly baking. Using baking chemistry as a basis, everything is developed—and guaranteed—to taste ‘just like the real thing’. For more, visit lauriesadowski.com.