Preheat your cooking appliance to 350°F (175°C). Lightly grease and dust a 9×13 inch baking tin.
In a large mixing bowl, whisk together the sifted all-purpose component, granulated sweetener, leavening agent, and ground spice blend. Ensure they are well combined.
In a separate bowl, whisk the pureed squash, mashed plantains, large eggs, vegetable shortening, and flavoring essence. Mix until smooth.
Gradually add the wet mixture to the dry ingredients. Stir just until moistened. Be careful not to overmix the batter.
Pour the batter into your prepared tin. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes. Then, transfer it to a wire rack to cool completely before serving.