Prepare the filling: Peel, core, and dice the gala apples into small pieces.

Cook the filling: In a sauce pot over medium heat, melt the salted butter and brown sugar, mixing them together until fully combined.

Combine ingredients: Stir in the diced apples, flour, and apple pie spice. Cover and reduce heat to medium-low, cooking until apples are softened, about 10 minutes.

Cool down: Once apples are tender, remove from heat and let the filling cool down completely.

Preheat & prepare pan: Preheat the oven to 350℉ (175℃) and grease a donut pan.

Mix wet ingredients: In a large bowl, whisk together the eggs, brown sugar, buttermilk, melted butter, and vanilla extract.

Incorporate dry ingredients: Sift together flour, baking powder, and baking soda into the wet mixture.

Add the filling: Fold in the cooled apple pie filling into the donut batter.

Fill the donut pan: Transfer the batter into a piping bag and fill the donut cups.

Bake donuts: Place the donut pan in the oven and bake for 11-12 minutes.

Cool down: Let the donuts cool on a wire rack for about 5 minutes.

Prepare the topping: Melt butter and mix granulated sugar with cinnamon.

Coat donuts: Brush each warm donut with melted butter, then dip into the cinnamon sugar mixture.