Preheat oven to 350 degrees. Lightly butter an 8-inch square baking pan and line with parchment paper, leaving a 2-inch overhang on all sides. Whisk together cornstarch, cocoa, cinnamon, and salt. In a large microwave-safe bowl, microwave butter and chocolate in 30-second increments, stirring each time, until melted and smooth, about 2 minutes. Stir in sugar and vanilla. Stir in eggs, one at a time, until combined. Add cornstarch mixture and stir vigorously until mixture is smooth and begins to pull away from side of bowl, about 2 minutes. Stir in pecans.
Pour batter into pan and smooth top. Bake until a toothpick inserted in center comes out with a few moist crumbs attached, about 35 minutes, rotating pan halfway through. Let cool completely in pan on a wire rack. Using paper overhang, lift cake out of pan and cut into 16 squares.