Step 1: Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Step 2: In a medium saucepan, combine water, 1/2 cup butter, and salt. Bring to a boil over medium-high heat.
Step 3: Add the flour all at once and stir vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan. Cook for 1-2 minutes to dry out the dough.
Step 4: Transfer the dough to a mixing bowl and let it cool for 5 minutes. Add eggs one at a time, beating thoroughly after each addition until the batter is smooth and glossy.
Step 5: Pipe or spoon 12 mounds of dough onto the prepared baking sheet. Bake for 20-25 minutes, then reduce heat to 350°F and bake for another 10-15 minutes until golden and firm.
Step 6: Poke a small hole in the side of each puff to let steam escape and let them cool completely on a wire rack.
Step 7: In a small skillet, melt 1 tbsp butter. Add the diced apples, brown sugar, and 1 tsp cinnamon. Sauté over medium heat for 5-8 minutes until tender. Let cool completely.
Step 8: In a chilled bowl, whip the heavy cream, powdered sugar, vanilla, and 1/2 tsp cinnamon until stiff peaks form.
Step 9: Cut the tops off the cooled puffs. Place a spoonful of the apple mixture into the bottom of each shell.
Step 10: Pipe the cinnamon whipped cream over the apples and replace the top of the pastry shell.
Step 11: Dust with a little extra powdered sugar if desired and serve immediately.
Step 12: Enjoy your homemade cinnamon apple cream puffs with a warm beverage.