Preheat oven to 350°F. Spray a 13x9-inch baking pan with vegetable oil cooking spray and set aside (5 minutes prep time).

In a large bowl, cream together 1/2 cup shortening, 3/4 cup packed brown sugar, and 1/2 cup sugar using an electric mixer on medium speed until the mixture is light, fluffy, and pale in color, about 2–3 minutes. Checkpoint: The mixture should look creamy and slightly increased in volume.

Add 2 eggs one at a time to the creamed mixture, beating well for about 30 seconds after each egg is added. The mixture should be smooth and well combined. Checkpoint: No streaks of egg should be visible.

Mix in 1 cup mashed ripe banana (about 2–3 medium bananas) and 1 teaspoon vanilla extract, stirring until fully combined, about 1 minute. The batter will appear slightly curdled—this is normal. Checkpoint: The banana is evenly distributed throughout.

In a separate small bowl, whisk together 2 cups all-purpose flour, 1 teaspoon baking soda, and 1 teaspoon salt until well combined, about 30 seconds. Checkpoint: No lumps of baking soda should remain.

Add half of the dry flour mixture (about 1 cup) to the banana mixture, stirring with a spatula until just combined, about 30 seconds. Do not overmix.

Add 1/2 cup buttermilk to the batter, stirring until just combined, about 30 seconds.

Add the remaining dry flour mixture (about 1 cup) to the batter, stirring until just combined, about 30 seconds. The batter should be smooth with no visible streaks of flour. Checkpoint: The batter is thick, creamy, and well combined.

Stir in 1/2 cup chopped nuts (walnuts or pecans work well) until evenly distributed, about 30 seconds. Checkpoint: Nuts are scattered throughout the batter.

Pour the entire batter into the prepared 13x9-inch pan, using a spatula to smooth the top into an even layer, about 1 minute.

Place the pan on the center rack of the preheated 350°F oven and bake for 25–30 minutes. The cake is done when a toothpick inserted near the center comes out clean or with just a few moist crumbs attached. The top should be light golden brown and spring back slightly when lightly touched. Checkpoint: A toothpick inserted in the center shows no wet batter.

Remove the cake from the oven and place on a wire cooling rack. Let cool completely in the pan at room temperature for at least 2 hours before frosting. This allows the cake to set and makes frosting easier. Checkpoint: The cake is completely cool to the touch.

While the cake cools, prepare the frosting. In a medium saucepan, combine 1/2 cup packed brown sugar, 1/4 cup butter, and 6 tablespoons milk.

Place the saucepan over medium heat and stir constantly until the mixture reaches a gentle boil, about 3–5 minutes. Checkpoint: The mixture is bubbling gently around the edges.

Once boiling, continue to stir constantly for exactly 2 minutes. The frosting will darken slightly and thicken. Checkpoint: A spoon dipped in the mixture coats slightly.

Remove the saucepan from heat and let the frosting cool at room temperature for about 5–10 minutes, stirring occasionally. The frosting should be warm but not hot when you add the powdered sugar. Checkpoint: You can hold your hand over the pan without it being too uncomfortable.

Gradually add 2 1/2 to 3 cups powdered sugar (start with 2 1/2 cups) to the cooled brown sugar mixture, beating with an electric mixer on medium speed. Add powdered sugar slowly to avoid lumps, about 1–2 minutes total. Checkpoint: The frosting is smooth, creamy, and spreadable—not too thick or runny.

If the frosting is too thin and won't hold its shape, add more powdered sugar 1 tablespoon at a time until it reaches the right consistency. If it's too thick, add milk 1 teaspoon at a time. Checkpoint: The frosting is thick enough to hold peaks but spreads easily on the cake.

Using an offset spatula or regular knife, spread the frosting evenly over the top of the completely cooled cake, working quickly before the frosting sets, about 2–3 minutes. Checkpoint: The frosting is smoothly applied and covers the cake evenly.

Allow the frosted cake to set for about 30 minutes to 1 hour before cutting and serving. This helps the frosting firm up slightly. Checkpoint: The frosting holds its shape when you press it gently.