Step 1

Preheat oven to 350°. Make a foil sling: Cut 18" length foil and fold lengthwise to 8" width. Fit foil into length of 13x9" baking pan (I recommend using a metal one), pushing into corners and up the sides of pan; allow excess to overhang pan edges. Cut 14" length foil and fit into width of pan in the same manner, perpendicular to the first sheet. Spray foil with nonstick cooking spray.

Step 2

In a large microwave safe bowl, whisk cocoa and boiling water together until smooth. Add unsweetened chocolate and whisk until the chocolate is melted. If you need to, place the bowl in the microwave in 30 second increments to help melt the chocolate.

Step 3

Whisk in the melted butter and oil. Mixture may look curdled at this point and that's okay! Add eggs, yolks, and vanilla extract, and continue to whisk until smooth and homogenous.

Step 4

Whisk in sugar until fully incorporated.

Step 5

In a small bowl, whisk together the gluten-free flour blend, xanthan gum (if needed), and salt.

Step 6

Stir the flour mixture into wet ingredients and mix with a rubber spatula until combined.

Step 7

Fold in the bittersweet or semi-sweet chocolate pieces.

Step 8

Scrape batter into prepared pan and bake until toothpick inserted halfway between edge and center comes out with just a few moist crumbs attached, 30 to 35 minutes. Transfer pan to a wire rack and cool 1 1/2 hours.

Step 9

When cool, cut and serve! The foil sling makes it easy to pull them out and place on cutting board for easy cutting. These cut very nicely with a plastic bladed cutter.

Step 10

If you use a glass baking dish, cool the brownies 10 minutes, then remove them promptly from the pan so they don't overbake in the hot pan.

Step 11

Store leftovers at room temperature.