Inside this classic cake is a sweet streak of brown sugar and warm cinnamon with the nutty crunch of walnuts. Chop the walnuts to the size of small peas to ensure they are evenly distributed in the cake. Substitute pecans for walnuts, if desired.
Combine brown sugar, walnuts, and cinnamon in a small bowl. Set aside.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking soda, baking powder, and salt in a medium bowl, stirring well with a whisk.
Combine sugar and butter in a large bowl; beat with a mixer at medium-high speed until well combined (about 3 minutes). Add egg substitute; beat 3 minutes or until combined. Beat in vanilla.
Add flour mixture and buttermilk alternately to sugar mixture, beginning and ending with flour mixture, beating well after each addition and scraping sides of bowl. Spoon half of batter into a 10-inch Bundt pan coated with cooking spray. Sprinkle half of brown sugar mixture evenly over batter; spoon remaining half of batter into pan. Top with remaining brown sugar mixture.
Bake at 350° for 55 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.