Preheat your oven to 350°F (175°C) and grease a 10-inch bundt pan.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Stir in the vanilla extract.
In another bowl, combine the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Fold in the diced apples and cinnamon (if using) gently until evenly distributed.
Pour the batter into the prepared bundt pan, smoothing the top.
Bake in the preheated oven for about 50 minutes, or until a toothpick inserted comes out clean.
Allow the cake to cool in the pan for 10 minutes before inverting onto a wire rack to cool completely.