Preheat your oven to 350°F (175°C). Grease and lightly flour an 8x8-inch square baking pan to ensure the cake doesn't stick.

In a medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt. Set these dry ingredients aside.

In a large mixing bowl, beat the softened butter and packed brown sugar together on medium speed until light and fluffy, about 2 to 3 minutes. Add the eggs one at a time, beating well after each addition, then stir in the vanilla extract.

Gradually add the dry flour mixture to the wet ingredients, alternating with the buttermilk. Mix on low speed until just combined, being careful not to overmix the batter.

Gently fold the diced apples into the batter using a rubber spatula until they are evenly distributed. Pour the thick batter into the prepared baking pan, smoothing the top into an even layer.

Bake in the preheated oven for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean and the top is a deep golden brown. Let the cake cool in the pan for at least 15 minutes before slicing, and serve warm with a scoop of vanilla ice cream or a dusting of powdered sugar.