Preheat the oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper and lightly grease it.
In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, salt, ground cinnamon, and ground nutmeg. Set aside.
In a large bowl, beat the softened unsalted butter, granulated sugar, and light brown sugar together until creamy and fluffy, about 2-3 minutes.
Add the eggs one at a time to the butter and sugar mixture, beating well after each addition.
Mix in the vanilla extract and mashed ripe bananas until fully combined.
Gradually add the dry flour mixture to the wet ingredients, alternating with the buttermilk, beginning and ending with the flour mixture. Mix until just combined; do not overmix.
Fold in the chopped walnuts gently with a spatula.
Pour the batter evenly into the prepared baking pan and smooth the top with a spatula.
Sprinkle the 1/4 cup granulated sugar evenly over the top of the batter to create a crisp snap-like crust.
Bake in the preheated oven for 22-25 minutes, or until a toothpick inserted into the center comes out clean and the top is golden and crisp.
Remove from the oven and allow the banana cake snap to cool completely in the pan on a wire rack.
Once cooled, lift the cake out using the parchment paper and cut into 12 squares.
Serve and enjoy your banana cake snap as a crispy, flavorful treat.