Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan to prevent sticking.

In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer on medium speed until light and fluffy, about 3-4 minutes. This incorporates air into the mixture, helping the cake rise.

Add the mashed bananas, eggs, and vanilla extract to the butter-sugar mixture. Mix until well combined.

In a separate bowl, whisk together the sifted flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, alternating with the cooled brewed coffee. Start and end with the flour mixture. Mix until just combined; do not overmix.

If using, fold in the chopped walnuts gently with a spatula.

In a medium bowl, combine the flour, brown sugar, and ground cinnamon. Add the cold, cubed butter.

Using a pastry cutter or your fingers, mix until the mixture resembles coarse crumbs. This will create a crunchy texture on top of the cake.

Pour the cake batter into the prepared cake pan, spreading it evenly.

Sprinkle the streusel topping evenly over the cake batter.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted in the center comes out clean. This ensures that the cake is fully cooked.

Once baked, remove the cake from the oven and allow it to cool in the pan for 10 minutes before transferring it to a cooling rack.

Allow the cake to cool completely before slicing. Serve warm or at room temperature with a cup of coffee or tea.