8 tablespoons unsalted butter (1 stick), cut into 8 pieces, plus more for coating the pan
4 ounces unsweetened chocolate, finely chopped
1/3 cup gluten-free oat flour
1/3 cup sweet rice flour
1 teaspoon xanthan gum
1 teaspoon baking powder
1/2 teaspoon fine salt
1 cup granulated sugar
2 large eggs
1 teaspoon vanilla extract
1/4 cup natural peanut butter, at room temperature (optional)
Heat the oven to 350°F and arrange a rack in the middle. Coat an 8-by-8-inch square baking dish with butter; set aside.
Fill a medium saucepan with 1 to 2 inches of water and bring to a simmer over high heat; once simmering, reduce the heat to low. Place the measured butter and chocolate in a large, dry, heatproof bowl. Set the bowl over (but not touching) the water and, using a rubber spatula, stir until the chocolate and butter have completely melted and the mixture is smooth. Remove the bowl from the saucepan and let the mixture cool to room temperature, about 20 minutes.
Meanwhile, sift the oat flour, rice flour, xanthan gum, baking powder, and salt together into a medium bowl, discarding any coarse bits left in the sifter; set aside.
When the chocolate has cooled, add the sugar and whisk to combine. Add the eggs and vanilla and whisk until smooth. Stir in the flour mixture with a rubber spatula until evenly combined. Pour the batter into the prepared baking dish and smooth out the top. If you’re using the peanut butter, drop heaping teaspoonfuls of it over the top of the batter. Drag a knife through the batter in a figure eight to create a marbled pattern.
Bake until a knife inserted into the middle comes out clean, about 20 to 25 minutes. Cool completely on a wire rack before serving.
*Recipe and image courtesy of CHOW.com.*