Preheat oven to 350°
Prepare apples by coring, peeling and chopping into ½" cubes. Toss in a tablespoon of lemon juice.
Prepare a 9" round cake pan by lining the bottom with parchment paper or greasing the pan.
Whisk flour, baking powder, salt and cinnamon in a small bowl.
Place butter and granulated sugar in bowl of a stand mixer. Using the paddle attachment, begin mixing butter and sugar until light and fluffy (2 - 4 minutes), scraping bowl occasionally. Note: a hand mixer may also be used.
Add room temperature eggs, one at a time, scraping bowl between each addition. Mix on medium-low speed until eggs are well incorporated.
Add rum and mix for approximately 30 seconds,
Add flour mixture and mix on low speed until just combined.
Stir chopped apples into cake batter by hand.
Pour cake batter into prepared pan and smooth the top lightly with spatula.
Sprinkle 2 tablespoons of granulated sugar evenly over top of cake.
Optional: Place apple slices in a spiral pattern on top of cake batter.
Bake for 40 - 50 minutes at 350°. The cake top will be a golden color and a toothpick inserted in the center of the cake will come out clean when the cake is done.
Place cake on a rack and allow to cool completely before unmolding (approximately 40 minutes).
If desired, sprinkle confectioners' sugar over top of cake after unmolding.