Take your regular brownie recipe. Pour the batter into the pan. Spoon the filling (recipe below) in dollops across the top and blend with a knife for the swirl effect. Bake as normal - it may need a wee bit longer. I also keep them refrigerated, due to the cream cheese.
*4 oz cream cheese
*2+ teaspoons of matcha, depending on how green you want it
*2 tsp sugar
Blend in mixer until smooth.
By the way, use food grade matcha for this. Matcha can be expensive and you want to save the more expensive tea grade for drinking. The less expensive food grade is perfectly fine for cooking.