Make the cupcakes: Preheat the oven to 350 degrees F. Line 10 standard-size (3-ounce) muffin cups with paper liners. (Liners are not optional—the cupcakes will stick, even with nonstick spray.)
Heat the chocolate, butter, and espresso powder in a medium saucepan over low heat, until the chocolate is almost completely melted. Remove from the heat and stir until the chocolate is completely melted and smooth. Let cool to room temperature, stirring occasionally.
In a medium bowl, beat the egg yolks and 3 tablespoons sugar with an electric mixer until the mixture is very thick and pale, about 2 minutes. (I found a hand mixer, rather than a stand mixer, worked best because the amounts are fairly small.) Add the cooled chocolate mixture and vanilla extract and beat briefly. Set aside.
Clean and dry your beaters COMPLETELY. In a separate medium bowl, beat the egg whites until soft peaks form. Gradually beat in the remaining 3 tablespoons sugar and salt, and continue beating until medium-firm peaks form. Fold the whites into the chocolate mixture one-third at a time, fully and gently incorporating between each addition. Divide the batter among prepared cups, filling each a little more than three-fourths of the way. Do not be tempted to overfill, as the cupcakes will not bake properly.
Bake until the tops are puffed and dry to the touch (some may crack, but they will still be delicious) and a toothpick inserted into the centers comes out mostly clean with some moist crumbs attached, about 15 to 20 minutes. Place the pan on a cooling rack and leave the cupcakes in the pan to cool completely. The cupcakes will start to fall almost immediately (perfect for the strawberry cream!).
Meanwhile, prepare the whipped strawberry cream cheese frosting: In a medium bowl, beat together the cream cheese and 3 tablespoons powdered sugar. Add the strawberries and food coloring (if desired) and beat until well blended. Set aside. In a separate bowl, beat the heavy cream and lemon juice together until foamy. Increase the speed to medium high and slowly add the remaining 2 1/2 tablespoons sugar, beating until stiff peaks form. Fold half of whipped cream mixture into the cream cheese mixture. Once that is combined, fold in the remaining half. Remove the cupcakes from the pan and place on a serving plate. Fill each sunken top with an extremely generous dollop of the strawberry cream cheese frosting and top with a slice of fresh strawberry. Dive in or refrigerate until ready to serve.
I know the recipe yield is a bit strange, and it may vary slightly based on the size of the cups in your particular muffin tin. Either over- or underfilling the liners does not work well. Fill them to slightly above three-quarters of the way as the recipe suggests and you will be golden.
YES, you can double recipe. In fact, you should probably go ahead and do it. You will not regret your decision. If you do double the cupcakes, I'd suggest multiplying the strawberry topping by about 1 1/2.
Store leftover cupcakes in the refrigerator for up to 3 days. I recommend adding the strawberry cream cheese topping just before serving (store it separately), but you can store them already topped too.