Flour: ¼ cup flour will make for a fudgier, sweeter brownie, while ⅓ cup will give you a cakier brownie.
Gluten-Free: Feel free to sub your favorite gluten-free flour blend for the pastry flour, if needed. While a flour blend with a little xanthan or guar gum will help it set up a little better, it's okay if you don't have a binder. Since you'll be eating it in the mug, a little crumbly doesn't hurt!