Preheat the oven to 350°F (175°C) and grease a muffin tin.

In a mixing bowl, whisk together flour, baking powder, baking soda, and salt.

In another bowl, mix sugar, milk, vegetable oil, egg, and vanilla until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

In a skillet over medium heat, cook the ground beef until browned.

Drain excess fat, then add taco seasoning and water, stirring to combine.

Simmer for 5 minutes until thickened.

Fill each muffin cup halfway with cupcake batter.

Add a spoonful of taco filling on top of the batter.

Top with remaining batter until cups are full.

Bake for 20-25 minutes or until a toothpick comes out clean.

Let cool slightly, then remove from the tin.

Top with sour cream, salsa, lettuce, tomatoes, olives, and jalapeños as desired.