Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing and flouring them.

Toast the walnuts in a dry skillet over medium heat for about 3-4 minutes until fragrant, then set aside to cool.

In a bowl, whisk together flour, baking powder, baking soda, and salt.

In a separate bowl, cream together butter and sugar until light and fluffy, then add eggs one at a time and mix until combined.

Stir in mashed bananas and buttermilk until just combined.

Gradually combine dry and wet mixtures, folding in toasted walnuts.

Pour batter into prepared pans and smooth the tops. Bake for 30-35 minutes, or until a toothpick comes out clean.

Let the cakes cool in pans for 10 minutes before transferring to wire racks.

While cakes cool, prepare banana filling by tossing banana slices with lemon juice.

Whip the cream with powdered sugar and vanilla until stiff peaks form.

Layer one cake on a plate, spread whipped cream, add sliced bananas, then top with the second cake and frost the top and sides.

Refrigerate the assembled cake for at least 1 hour before serving.