Preheat oven to 350℉ (175℃) and grease your bundt pan.
Sift the flour, baking soda, baking powder, and salt into a medium mixing bowl. Set aside.
Mash your bananas and add the lemon juice.
Whip the butter, vegetable oil, sugar, and brown sugar in a separate bowl on medium-high until pale and creamy.
Add eggs one at a time, mixing well after each addition. Then add vanilla, lemon zest, and mashed bananas.
Slowly combine the dry and wet ingredients in three batches, alternating with the buttermilk but beginning and ending with the flour. Stir until just combined.
Bake at 350℉ (175℃) for 45-50 minutes until a cake tester inserted into the cake's center comes out clean.
Cool completely and proceed with the glaze.
Bring the butter, brown sugar, and milk to a simmer in a small saucepan, whisking frequently, and cook until the sugar dissolves (a minute or two).
Remove from the heat and let it cool for 5 minutes. Whisk in the vanilla extract.
Whisking vigorously, add the powdered sugar ¼ cup at a time until you achieve the desired consistency.
The glaze will firm up rapidly, so ensure the cake is cooled and ready to be glazed. Drizzle the glaze over the cake and then top immediately with nuts if desired.