Preheat oven to 350 degrees Fahrenheit. Prepare 12-Cup Bundt Pan with butter and a light coating of flour. Tap out excess flour and set aside.
Mix all cake ingredients except walnuts in stand mixer on medium speed until smooth. Fold in walnuts and transfer batter to prepared bundt pan.
Bake for 45-60 minutes until toothpick inserted into the center comes out clean. Cool for 15 minutes before inverting onto a wire rack to cool completely.
To make the frosting, combine brown sugar, milk and butter in a small saucepan. Stirring constantly, cook over low heat until sugar has just dissolved. Add powdered sugar and stir until smooth.
Remove from heat and spoon over cooled cake. Top with additional walnuts if desired.
Recipe by Bake Love Give at http://bakelovegive.com/picnic-cake/