Start by making the ganache. Place the butter and milk in a bowl and microwave until bubbling. Break up the milk chocolate into the bowl, then whisk gently until smooth and glossy, then set aside to cool.
Preheat the oven to 200C/390F (180C/355F fan).
Put all of the sponge ingredients in a bowl and beat with an electric mixed until smooth.
Spoon or pipe the batter evenly into 10-12 mini loaf cases or a mini loaf tin (you'll get 10-12 depending on how large your tins/cases are).
Bake for 8-10 minutes until just firm on top.
Cool on a wire rack, remove the cases if necessary, then using a sharp knife, slice in half to create sandwiches.
To make the caramel, place the butter, sugar, vanilla and condensed milk in a pan over a medium heat and warm, stirring regularly until it thickens and darkens. Take off the heat and allow to cool for a couple of minutes.
Spoon 2-3 tsp of caramel onto the lower halves of each of the cakes and replace the tops to make sandwiches. The caramel will ooze out as it's still warm, but that's half the fun. It'll firm up as it cools, so they're not as messy to eat as you might expect.
Spoon the ganache over each of the cakes, allowing it to drip down the sides.
Top with sprinkles, if desired. I used gold candy sprinkles, because they're a millionaire's loaves, after all.
Ultra decadent millionaire's mini loaves https://www.amummytoo.co.uk/millionaires-mini-loaves/