To prepare the filling:

1. Using a blender or food processor, puree the guava shells and their juice until smooth. Add lemon juice and blend to combine.
2. In s small saucepan over low heat, cook while stirring occasionally until the mixture thickens, about 10 minutes. Set aside and allow to cool.

To prepare the cake:

1. Preheat oven to 350 degrees F.
2. In the bowl of a stand mixer fitted with a paddle attachment, cream the butter, granulated sugar, brown sugar and eggs on medium speed. Reduce the mixer speed to low, and beat in vanilla and almond extracts. Blend in the flour in small batches allowing each to fully combine.
3. Lightly grease a 9-inch round springform pan. Add half the batter to the bottom of the pan. Use an offset spatula to evenly spread the guava filling. Top the guava filling with the remaining cake batter.
4. Bake until a cake tester comes out clean, about 40 minutes.
5. Allow cake to cool in pan for 10 minutes. Run an offset spatula around the edge to loosen the cake from the pan. Remove the locking ring, and allow cake to fully cool. Carefully transfer to a serving platter.