Preheat oven to 350°F. Spray an 8-inch cake pan with baking spray and line the bottom with parchment paper.

In a clean bowl, beat egg whites until stiff peaks form. Set aside.

In another bowl, beat egg yolks for 1 minute. Gradually add brown sugar and beat until pale and fluffy.

Mix in melted white chocolate and oil. Stir in vanilla extract.

Fold in almond flour and pistachios. Add green food gel if using.

Fold in whipped egg whites until no streaks remain.

Transfer batter to the pan and bake for 30 minutes or until a toothpick inserted comes out clean.

Cool in pan for 5 minutes, then transfer to a wire rack. Garnish and serve.