Preheat oven to 350°F. Combine butter, brown sugar and flour and stir until the mixture is crumbly. Press mixture into an 8 inch greased baking pan. Bake for 10 minutes. Remove and set aside.
In a saucepan, combine the maple syrup, brown sugar and walnuts and bring to a boil. Reduce heat and cook over medium-low heat for 5 minutes. Remove from heat and add butter and stir until melted.
In a bowl, beat the eggs with the flour and salt. Add egg mixture to maple syrup mixture and stir to combine. Pour over the base and bake for another 25 to 30 minutes, or until set. Cool completely before cutting.