In a microwave-safe bowl, combine 1/2 cup of unsalted butter and 4 oz of semi-sweet chocolate. Microwave in 30-second increments, stirring after each, until smooth and fully melted, about 90 seconds total. Set aside to cool slightly.

In a separate mixing bowl, crack 2 large eggs and beat until frothy. Gradually add 1/2 cup of sugar, beating until dissolved and mixture is pale and slightly thick, about 2-3 minutes. Gently mix in the melted chocolate mixture until well combined.

Sift together 1/4 cup of all-purpose flour, 1/2 teaspoon of baking powder, a pinch of salt, and 1 teaspoon of vanilla extract. Add this dry mixture to the chocolate-egg mixture and stir gently until just combined, ensuring a smooth batter free of lumps.

Preheat your air fryer to 375°F (190°C). Generously grease four 4 oz ramekins with butter. Fill each ramekin with batter about three-quarters full.

Carefully place the filled ramekins in the air fryer basket without touching. Cook for 7 minutes for a gooey center or up to 9 minutes for a firmer texture. Small cracks during baking are normal.

Once baking time is up, gently remove the ramekins using oven mitts. Allow them to cool for about 2 minutes before inverting onto a plate. Serve immediately while warm for best molten experience, with optional toppings.