Preheat the oven to 350°F (175°C). Grease a muffin tin or line with paper liners.

For the caramel layer, combine the granulated sugar and water in a small saucepan over medium heat. Stir constantly until the sugar dissolves, and then allow it to cook until it turns golden brown. Quickly pour the caramel into the bottom of each muffin cup, swirling to coat evenly. Set aside.

For the flan batter, whisk together the eggs, sweetened condensed milk, whole milk (or evaporated milk), and vanilla extract in a bowl. Pour a small amount of the flan batter over the caramel layer in each muffin cup.

For the muffin batter, whisk together the flour, baking powder, baking soda, and salt in a separate bowl. In another bowl, beat the butter and sugar together until light and fluffy. Add the egg and vanilla extract, and continue mixing until smooth. Gradually add the dry ingredients and buttermilk, mixing until just combined.

Spoon the muffin batter over the flan batter in each muffin cup, filling about 2/3 of the way full.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Let the muffins cool in the tin for about 10 minutes, then transfer to a wire rack to cool completely.