Prepare the caramel: In a small saucepan, combine sugar and water. Heat over medium heat without stirring until it turns a golden amber colour. Immediately pour into the bottom of a greased muffin tin (or silicone muffin cups), swirling gently to coat the bottom. Set aside to cool and harden.
Make the flan layer: In a bowl, whisk together the eggs, sweetened condensed milk, whole milk, and vanilla until smooth. Pour about 2 tablespoons of this flan mixture over the hardened caramel in each muffin cup.
Preheat the oven: Preheat to 175°C (350°F). Prepare a hot water bath by placing the muffin tin inside a larger baking dish. Fill the outer dish with hot water halfway up the sides of the muffin tin.
Prepare the muffin batter: In a large bowl, cream butter and sugar until light and fluffy. Beat in the eggs one at a time, then add vanilla. In a separate bowl, whisk flour, baking powder, and salt. Gradually add dry ingredients to the butter mixture, alternating with milk, until fully combined.
Assemble: Carefully spoon a layer of the muffin batter over the flan in each cup, filling no more than 3/4 full.
Bake: Bake in the water bath for 45–50 minutes or until the cake layer is golden and a toothpick inserted into the muffin comes out clean.
Cool and invert: Allow to cool completely at room temperature, then refrigerate for at least 1 hour. To serve, gently run a knife around the edges and invert each muffin onto a plate to reveal the caramel top.
Tips
Use silicone muffin moulds or grease metal tins thoroughly to ensure easy release.
Let them chill overnight for a firmer flan texture.