Place a large heatproof glass bowl over a simmering pot of water (the water should not tough the bowl). Add butter and chocolate to the bowl. Stir the chocolate, until it melts. Remove the bowl and set aside.
In a medium sized mixing bowl, add sugar to it. Pour in the melted chocolate mixture. Whisk sugar into the chocolate. Then, add eggs and whisk. Add the espresso powder and salt and whisk until the batter is smooth.
Add the flour and fold in with a rubber spatula.
Pour the batter evenly into each heart in the baking pan. Place on a baking sheet with an OXO baking mat or covered in foil. Bake for 18 to 22 minutes. (The cake shouldn’t wobble or jiggle when it’s done baking.)
Place baking pan on a cooling rack for 3 to 5 minutes.
Place a piece of parchment paper on the cooling rack and put on top of the pan. Carefully flip the pan over and pick up the pan, the heart shaped chocolate cakes should come out easily.
Serve cakes topped with vanilla ice cream and sprinkles.
I love to serve these little heart shaped chocolate cakes warm. Before storing them in a sealed container, be sure to let them cool completely. You can warm them back up in the microwave for 15 to 30 seconds (but do this in 10 to 15 second increments to make sure they don't burn).