Preheat oven to 350°F with rack in middle. Butter and flour a 13- by 9-inch baking pan.
Melt butter, peanut butter, and chocolate in a 3-qt heavy saucepan over low heat, stirring, until smooth. Remove from heat and cool. Whisk in brown sugar and vanilla. Whisk in eggs 1 at a time.
Whisk together flour and salt, then whisk into chocolate mixture.
Spread in pan. Bake 25 minutes.Sprinkle with brittle. Bake until a pick inserted in center comes out with crumbs, about 5 minutes. Cool completely.