• Begin by preheating your oven to 325°F. Prepare two muffin tins by greasing them with butter and dusting with flour, or line them with paper liners.

For Topping:

• In a mixing bowl, combine the brown sugar, all-purpose flour, unsalted butter, and cinnamon until you achieve a crumbly texture. Set the mixture aside.

For Muffin Batter:

• In a large bowl, blend together the brown sugar, vegetable oil, egg, and vanilla extract until smooth.
• In a separate bowl, mix the water and apple cider vinegar together.
• In another bowl, sift the all-purpose flour, baking soda, and salt together.
• Gradually combine the flour mixture with the water-vinegar mixture, alternating with the sugar mixture until fully incorporated.
• Gently fold in the diced apples until evenly distributed throughout the batter.
• Fill each muffin cup about three-quarters full with the batter.
• Sprinkle the crumb topping over each muffin generously.
• Bake for approximately 30 minutes, or until the tops are golden brown and a toothpick inserted in the center comes out clean. The muffins should spring back when touched.
• After baking, allow the muffins to cool slightly in the pan before transferring them to a wire rack to cool thoroughly.