Heat the oven to 400°F. Coat 6 muffin cups with non-stick cooking spray.
Combine flour, brown sugar, and butter in a medium bowl. Cut in butter with a pastry blender until the mixture is crumbly. Stir in walnuts.
Stir muffin mix and milk in a medium bowl just until evenly moistened. Divide evenly into muffin cups. Top evenly with streusel mixture.
Bake for 14 to 17 minutes or until a toothpick inserted in the center comes out clean. Cool in the pan for 3 minutes. Remove to a wire rack to cool slightly.