1⁄/2 cup unsalted butter
1⁄/2 cup chocolate chips
3 large eggs
1⁄/2 cup sugar
1 teaspoon pure vanilla extract
Generously grease a 7-inch square baking pan with vegetable oil.
In a microwave-safe bowl, combine the butter and chocolate chips. Microwave on high for 1 minute. Stir very well. (You want the heat from the butter and chocolate to melt the remaining clumps. If you microwave until everything melts, the chocolate will be overcooked. If necessary, microwave for an additional 10 seconds—but stir well before you try that).
In a medium bowl, combine the eggs, sugar, and vanilla. Whisk until light and frothy. While whisking continuously, slowly pour in the melted chocolate in a thin stream and whisk until everything is incorporated.
Pour the batter into the prepared pan. Set the pan in the air-fryer basket. Set the air fryer to 350°F for 25 minutes, or until a toothpick inserted into the center comes out clean.
Let cool in the pan on a wire rack for 30 minutes before cutting into squares.