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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Coat an 8-by-8-inch square baking dish with butter; set aside.
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3
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.
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4
Place the measured butter and chocolate in a large, dry, heatproof bowl.
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5
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.
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6
Remove the bowl from the saucepan and let the mixture cool to room temperature, about 20 minutes.
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7
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.
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8
When the chocolate has cooled, add the sugar and whisk to combine.
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9
Add the eggs and vanilla and whisk until smooth.
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10
Stir in the flour mixture with a rubber spatula until evenly combined.
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11
Pour the batter into the prepared baking dish and smooth out the top.
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12
If youre using the peanut butter, drop heaping teaspoonfuls of it over the top of the batter.
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13
Drag a knife through the batter in a figure eight to create a marbled pattern.
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14
Bake until a knife inserted into the middle comes out clean, about 20 to 25 minutes.
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15
Cool completely on a wire rack before serving.
