Preheat oven to 350°F with rack in middle. Butter and flour a 13- by 9-inch baking pan. Melt butter and chocolate with espresso powder in a 3-quart heavy saucepan over low heat, whisking until smooth. Remove from heat and cool to lukewarm. Whisk in sugar and vanilla. Whisk in eggs 1 at a time until mixture is glossy and smooth. Whisk together flour, cinnamon, and salt, then whisk into chocolate mixture. Spread batter in pan and bake until a wooden pick inserted in center comes out with crumbs adhering, 25 to 30 minutes. Cool completely before cutting.