Preheat oven to 350°F. Line an 8-by-8-inch pan with aluminum foil, leaving an overhang on two sides to lift the bars out later, and spray the foil lightly with cooking spray. Set the pan aside.

Place ¼ cup unsalted butter (half of one stick) in a large microwave-safe bowl. Microwave on high until melted, about 1 minute. Stir and let the butter cool 1–2 minutes so it is warm but not hot.

Add 1 large egg to the cooled butter and whisk until fully combined.

Add ½ heaping cup sweetened condensed milk, ½ cup lemon juice, 1 teaspoon vanilla extract, and 1 teaspoon lemon extract if using. Whisk until the mixture is smooth.

Add ¾ cup all-purpose flour, 2 teaspoons lemon zest, and ¼ teaspoon salt if using. Stir until the flour is just incorporated and there are no large streaks of dry flour.

Gently fold in 1 cup maraschino cherries that have been roughly chopped and patted dry.

Transfer the batter to the prepared pan and spread it into an even layer.

Bake for 19–23 minutes, or until the center is just set (a slight jiggle is okay). Avoid overbaking, which will make the bars rubbery.

Remove the pan from the oven and let the bars cool in the pan for at least 15 minutes.

While the bars are baking or cooling, melt 2 to 4 tablespoons white chocolate chips (if using) in a small microwave-safe bowl: heat in 15-second increments, stirring after each burst, until smooth. If needed, use 50% power to prevent scorching.

After the bars have cooled at least 15 minutes, lift them from the pan using the foil overhang and place on a wire rack or cutting board. Drizzle with the melted white chocolate, if using, and wait a few minutes for the drizzle to set.

Slice into bars and serve. Store in an airtight container at room temperature for up to 5 days, or freeze for up to 3 months.