Heat oven to 350°F. Line a 13 x 9 x 2-inch baking pan with nonstick foil.
In a large bowl, stir together brownie mix, egg, 1/4 cup water, oil and extract just until smooth. Stir in coconut and nuts. Spoon into prepared pan, spreading to edges.
Bake bar at 350°F for 20 to 25 minutes, or until center is set.
In a small saucepan, combine sugar, butter, evaporated milk and egg yolks. Cook over medium-high heat, whisking, until very bubbly and thickened, 13 to 15 minutes. Stir in coconut, nuts and vanilla; cook 2 more minutes.
After removing bar from oven, pour warm frosting over top, spreading to edges. Cool 20 minutes on wire rack at room temperature. Chill slightly, or until ready to serve. Use foil to lift bar from pan for easier slicing.