To prepare a sugarless banana walnut cake recipe, first you need to prepare your ingredients. First, you'll need: 1 cup (240 ml) unsalted butter, 1 tablespoon (2.5 g) brown sugar, and 1 teaspoon vanilla extract. Then, add the dry ingredients to the wet ingredients and mix until combined. Next, pour the batter into the prepared cake pan and bake the cake for 35 to 40 minutes, or until the toothpick inserted in the center comes out clean. If you don't have time to bake your cake right away, store the leftover cake in an airtight container for up to 3 days.