Use a hand-mixer to cream together the 4 tbsp softened butter, granulated sugar, and light brown sugar in a large bowl until light and fluffy.
Beat in the egg, the 1 tsp vanilla extract, and the reserved pineapple juice until combined.
In a separate medium bowl, whisk together the flour, baking powder, and salt.
Alternate adding the dry ingredients and the milk into the butter mixture, beating on low between each addition until ingredients are just combined.
Carefully pour the cake batter over the arranged pineapple, cherries, and nuts in the cake pan. Smooth batter to the edges of the pan to ensure an even cake layer.
Bake the Cinnamon Pineapple Upside-Down Cake at 350F for about 40 minutes, or until a toothpick inserted into the middle of the cake comes out clean.
Remove Cinnamon Pineapple-Upside Down Cake from the oven and let cool in pan on a wire track for ONLY 5 MINUTES.
Run a knife around the edges of the pan to loosen the sides of the cake.
Using oven mitts, cover the cake with the bottom of your cake plate/stand. Securely hold the cake plate against the baking pan, then quickly but carefully flip the cake over so that the pineapple "bottom" is now the topping of the cake. Carefully lift the cake pan up and away from the cake. Let Cinnamon Pineapple Upside-Down Cake cool completely before serving.