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Preheat oven to 350F. Line an 8x8-inch pan with parchment paper and spray with oil.
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Combine canola oil, butter, and chocolate in the top of a double boiler. Heat over simmering water until chocolate is almost full melted, stirring with a rubber spatula. Remove from heat and continue stirring until completely melted.
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Pour chocolate mixture into a large bowl, add sugar and stir until combined. Add 2 eggs, stirring. Stir in vanilla and salt. Fold in flour in two or three batches. Pour mixture into prepared pan.
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In a separate bowl, combine tahini, brown sugar, and remaining egg. Drop spoonfuls of tahini mixture onto brownie batter and swirl with the tip of a knife.
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Bake for about 30 minutes or until a wooden pick inserted in the center comes out mostly clean. Cool in pan on a wire rack for at least 15 minutes before serving.