Preheat Oven: Preheat your oven to 325°F (160°C) and line the bottom of an 8-inch square baking pan with parchment paper. Lightly spray the sides of the pan with cooking spray.
Cream Butter and Sugar: In a medium bowl, cream together softened butter and granulated sugar until light and fluffy, about 1-2 minutes.
Add Eggs and Vanilla: Beat in eggs, one at a time, then add vanilla extract and mix until combined.
Prepare Dry Ingredients: In a separate bowl, whisk together matcha powder, all-purpose flour, and baking powder.
Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet ingredients, alternating with buttermilk, and mix until just combined. Be careful not to overmix.
Pour Batter: Pour the batter into the prepared pan and spread it evenly.
Bake: Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out mostly clean with only moist crumbs.
Cool Cake: Let the cake cool in the pan for about 15 minutes, then transfer it to a wire rack to cool completely.
Slice Cake: Once cooled, slice the cake in half horizontally to create two rectangles.