Prepare the Cake
Preheat your oven to 350°F (175°C). Grease a 9x13-inch sheet pan and line with parchment paper if desired.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large bowl, mix the melted butter, granulated sugar, and brown sugar until smooth.
Beat in the eggs, one at a time, followed by the vanilla extract.
Gradually add the dry ingredients to the wet mixture, alternating with the milk, starting and ending with the dry ingredients. Mix until just combined.
Gently fold in the diced apples.
Pour the batter into the prepared pan and spread evenly.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool slightly in the pan.
Make the Salted Caramel Glaze
In a saucepan over medium heat, melt the butter. Add the brown sugar and heavy cream, stirring constantly.
Bring the mixture to a boil and let it simmer for 2-3 minutes.
Remove from heat and stir in the vanilla extract and salt. Allow the glaze to cool slightly.
Assemble the Cake
While the cake is still warm, pour the salted caramel glaze over the top, spreading it evenly. Let the glaze soak into the cake.
Allow the cake to cool completely before slicing and serving.