Microwave butter and chocolate in a large microwave-safe bowl at 50% power until melted, about 1 minute.
¼ cup butter, 1 ounce bittersweet chocolate
Stir in sugar, eggs, cocoa, vanilla, and salt.
¼ cup granulated sugar, 2 large eggs, 2 tablespoons unsweetened cocoa powder, 1 teaspoon vanilla, ¼ teaspoon salt
Add flour and baking powder, and whisk in.
¼ cup all-purpose flour, ½ teaspoon baking powder
Divide the batter between 2 12-16 ounce microwave-safe mugs.
Place mugs on microwave turntable.
Microwave at 50% power for 45 seconds.
Stir batter and microwave for another 45 seconds at 50% power.
Press 2 pieces of chocolate into the center of each cake. No need to press down. They should sink in on their own.
1 ounce semisweet or bittersweet chocolate
Microwave at 50% power for 60-90 seconds.
Cake should pull away from the edge of the mug when done.
Let sit 2 minutes before serving.
Notes
Bittersweet chocolate: Can substitute semisweet chocolate chocolate.
Flour: It's important to properly measure the flour. Either weigh the flour, or sift or stir it, lightly spoon into measuring cup, and then level. Too much flour will give you a dry or dense cake.
Microwaving at full powder: This will take less time. However, the chocolate will get absorbed into the cake, and you won't get a molten center.
Mugs: I like to use a 16-ounce mug because it allows more room for the cake to expand. A 12-ounce mug will work, but keep an eye on it to make sure it doesn't spill over as it cooks.
Microwave vary in wattage, and cooking times will vary according to the wattage.