Cut a piece a parchment paper to fit the bottom of a 10-inch, spring-form cake pan. Lightly grease the sides of the pan with butter and set aside.
Place the lemon juice in a medium bowl. Peel the apples, cut each one in half and scoop out the core. Set the apple halves in the bowl and coat with lemon juice. Cover and set apples aside.
Preheat the oven to 350 degrees F. Place the flour, almonds, baking powder, cinnamon, cocoa and salt in a second bowl and whisk to combine. Place the 1 cup butter and granulated sugar in the bowl of your stand mixer and beat until lightened. Beat in the eggs, one a time. Mix in the vanilla. Mix the dry ingredients into the wet until just combined. Spoon and spread the dense batter into the cake pan.
Set a half apple, curved side up, on a work surface. Cut thin, deep slices into the apple, but don't cut entirely through it. Repeat with the remaining apples. Arrange the apple halves, cut side up, on the cake, gently pushing on the apples until they are about halfway into the batter.
Bake the cake in the middle of the oven for 90 to 95 minutes, or until cake tester inserted in the centre of the cake pulls out clean. Cool the cake to room temperature, and then remove the pan's outer ring. Tightly wrap and keep the cake at cool temperature until needed. (This moist cake can be made a day or two before needed.) If desired, dust the cake with icing sugar just before serving.