Preheat toaster oven on the "Bake" function to 400 degrees.
Line the bottom of 4 individual tartlet pans with parchment paper and lightly spray the sides with oil like Bake-Kleen. Place the prepared pans on a quarter sheet pan or other size that fits in your toaster oven. If your oven is small you may need to bake in two batches.
In a medium microwave-safe bowl melt the chocolate and butter in a microwave in 20-second intervals, stirring in between melted and smooth. My microwave is a small 750-watt version and it takes me 2 minutes using 20-second intervals to completely melt the chocolate. Make sure all the chocolate is melted and smooth.
When melted remove from microwave and set aside.
While the chocolate is cooling, beat the eggs on medium-low until frothy, then beat high speed until the eggs are light and fluffy and have tripled in volume.
Stir in the flour and espresso powder into the chocolate mixture.
Fold the chocolate into the egg mixture.
Pour the batter dividing evenly, into the tartlet pans and bake for 15 minutes. The top should be firm and the middle will be soft like a mousse while the outer edges are firm,
Cool the cake in the tartlet pans for 30 minutes before removing it. Chill slightly before serving.