Lightly grease the bottom and sides of a 6-quart slow cooker. Line the bottom with parchment paper for easy removal, then grease the parchment as well. Alternatively, if you have a small springform pan that fits inside, use that for a round cake.

In a medium mixing bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, nutmeg, and allspice. Set aside.

In a large mixing bowl, mash the ripe bananas until smooth. Add the melted butter, oil, granulated sugar, and brown sugar, mixing until well combined. Beat in the eggs one at a time, followed by the buttermilk and vanilla extract until just incorporated. The mixture may look slightly curdled— that's okay.

Gently fold the dry ingredients into the wet mixture in three additions, stirring only until just combined to avoid overmixing. Fold in the optional walnuts if using.

Pour the batter into the prepared slow cooker. Smooth the top slightly with a spatula. Cover and cook on HIGH for 2-2.5 hours or LOW for 4-5 hours, until a toothpick inserted in the center comes out clean. Note: Cooking times can vary by slow cooker brand, so start checking at 2 hours on HIGH or 4 hours on LOW.

Remove the lid and let the cake cool in the slow cooker for 10 minutes. Carefully lift out the cake onto a wire rack (using the parchment if lined) and cool completely before frosting.

While the cake cools, prepare the cream cheese frosting. In a large mixing bowl, beat the softened cream cheese and butter together until smooth and lump-free, about 2 minutes with an electric mixer on medium speed.

Add the vanilla extract and optional lemon juice. Gradually add the sifted powdered sugar, one cup at a time, mixing on low until combined, then increase to medium-high and beat until light and fluffy, about 2-3 minutes.

Once the cake is completely cooled, spread the frosting generously over the top. Garnish with extra nuts or banana slices if desired. For best results, chill for 30 minutes before slicing to set the frosting.