1. Preheat the oven to 375°F. Coat a regular-sized muffin tin with nonstick cooking spray.
2. Place dark chocolate chips and butter in a microwave-safe bowl. Microwave for 30 seconds. Stir. Continue to microwave in 10-second increments, stirring in between, until chocolate is melted and smooth. This took a total of 1 minute in my microwave.
3. Add sugar to the melted chocolate and stir until well-mixed. Let cool for 2-3 minutes. Cooling prevents the eggs from cooking when they are added in the next step.
4. After a couple of minutes of cooling, stir in the beaten eggs, vanilla, and salt. Whisk until mixture slightly thickens and turns smooth and glossy.
5. Divide the mixture evenly into the prepared muffin tins. The batter filled my muffin tin cups about halfway full. 
6. Bake for 18 minutes, or until the tops are set and a toothpick inserted into the center comes out clean. The cakes will expand in the oven and then condense as they cool. Let the mini cakes cool for about 10 minutes before removing them from the muffin tin to a plate or sealable container. 
7. Serve warm dusted with optional powdered sugar and strawberry slices. Or, (my favorite!) cool the mini cakes completely, place them in an airtight container, and refrigerate until cold. I like to chill them overnight. Serve chilled with a sprinkle of optional powdered sugar and strawberry slices.

Storage: Cool mini cakes to room temperature. Refrigerate in a sealed container for 2-3 days. Freeze for up to 3 months. When ready to eat, thaw in the refrigerator and dust with optional powdered sugar right before serving.