Prepare the ramekins or mugs by greasing them with butter and dusting them with cocoa powder.
Heat the cream on the stove or in the microwave in a bowl until it is steaming and just shy of boiling. Add the chocolate chips for the ganache to a medium mixing bowl and pour the hot cream over them. Cover and let them sit for a few minutes.
Once the chocolate chips have melted completely, mix the cream and chocolate together until smooth. If the hot cream doesn't melt the chocolate completely, use the microwave in 15-second increments to finish melting the chocolate.and refrigerate it until it is firm.
In a large mixing bowl, add the butter and the remaining semi-sweet chocolate. Melt the chocolate and butter either over a double boiler or in the microwave. If you use a microwave, use low power and 30-second increments.
Add the powdered sugar, vanilla, and salt to the batter and mix. Then, slowly add the eggs and egg yolks to the batter. Be sure to use room-temperature eggs. Otherwise, the cold eggs could cause the batter to curdle and break. If you notice the batter curdling, see the notes below to fix it.
Mix in the flour until smooth.
Fill four 6-ounce ramekins about ½ way with the cake batter (see the notes below for other size dishes). Then, add a dollop of the cooled, firm ganache to the center of the ramekin. Then, fill the ramekin the rest of the way with the batter. You will want to leave about 1 or 2 inches of space in the mugs because the cakes will rise when cooked.
Bake the lava cakes in the air fryer at 360°F for 8-10 minutes or until the edges are firm and the center is still soft.
Let the cakes cool for 1-2 minutes, and use a butter knife to loosen the cake from the ramekins. Flip the ramekins over on a plate. You may need to give them a tap to release the cake onto the plate. Serve the lava cakes with fresh berries, ice cream, or whipped cream.