After cutting the apples into 1/8 in slices, run your knife through the pieces a few times, creating bite-sized pieces. Place the apples and the apple cider in a large saucepan over medium heat. Cover the pan and let the apples steam in the cider until tender, 8 to 10 minutes. The apples should be soft, but still hold their shape. Strain the apples and let cool to room temperature.
In a medium saucepan over medium heat, melt the butter, brown sugar, and salt. Remove from the heat and stir in the vanilla and apple jack, if using. Let the mixture cool to room temperature.
Add the eggs and whisk until combined. Add the baking powder and whisk again until incorporated, then add the flour and use a spatula to stir until combined. Add the cooled, strained apple pieces and stir to combine.
Transfer the batter to the prepared pan and use an offset spatula to smooth out the top. Sprinkle the cake evenly with the cinnamon-sugar mixture.
Bake for 24 to 30 minutes, until a wooden skewer or toothpick inserted into the batter comes out with the faintest hint of crumb.
Transfer the pan to a wire rack and let cool completely. Use the parchment sling to gently lift the cake out of the pan. Cut them into squares and serve. Cake can be stored in an airtight container at room temperature for up to 2 days.