Preheat your oven to 350°F (175°C) and grease a cake pan.

In a mixing bowl, cream together butter and sugar until light and fluffy.

Add eggs one at a time, beating well after each addition.

Mix in the vanilla extract and milk until fully combined.

In another bowl, combine flour, baking powder, salt, and cinnamon (if you're using it). Gradually add this dry mixture to the wet mix.

Gently fold in the diced apples and chopped walnuts, being careful not to over-mix.

Pour the batter into the prepared cake pan and smooth the top with a spatula.

Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean.

Once baked, let the cake cool in the pan for a few minutes before transferring it to a wire rack to cool completely.