Preheat your oven to 160 °C (320 °F) and line an 8" (20cm) square pan with parchment paper overlapping the sides.Cube 115 g unsalted butter and place it in a heatproof bowl. Microwave uncovered for 1 minute at 600W so that it's mostly (but not completely) melted. It should still be a little pale and creamy. (Alternatively, melt half of the butter in a pot on the stove top, then stir in the other half for similar results.)
Add 100 g light brown sugar and 50 g granulated sugar to the melted butter and mix until well combined.
Crack 1 egg into the bowl, and whisk until glossy. Then add 1 egg yolk and ½ tsp vanilla essence. Whisk and set aside.
Sift 130 g all-purpose flour and 6 g matcha powder into a separate bowl. Stir in ⅛ tsp salt.
Pour all of the dry ingredients into the wet ingredients, and fold together until no more pockets of dry flour remain.
Add 50 g filling of choice. I used a roughly chopped bar of semi sweet chocolate. Save 1-2 tbsp to sprinkle over the top.
Scrape the batter into the lined square pan, using the spatula to spread it evenly and push it into the corners. Sprinkle the top with the leftover mix-ins.
Bake for 18-20 minutes, or until the edges are slightly puffy and the center is no longer wet. If testing with a toothpick, it should come out with moist crumbs. Wet batter means it needs longer, clean toothpick means over baked.
Remove from the oven and cool in the pan for 10-15 minutes, then carefully tilt the pan and slide the blondies onto a wire rack. Avoid lifting while warm, this could break it.
For an extra fudgy texture, chill for a few hours. Cut into 12-16 equal pieces and enjoy!
Notes
For best results, use matcha that is bright green and smells good.
Weigh your ingredients for accuracy. If using cups, fluff the flour, then spoon and level to make sure you don't add too much.
Mix until just combined, over-mixing can make them tough or cakey.
These matcha blondies are about 1.5cm (about ⅔ inch) if baked in a 8" (20cm) pan. Make them thicker by using a 7" pan and increase the baking time by 3-5 minutes.
Be careful not to over bake, take them out as soon as the edges are coming away from the pan and the middle doesn't wobble. A toothpick should come out with moist crumbs and melted chocolate (a clean toothpick usually means overbaked).
Cool completely before cutting.
Chill in the fridge for extra fudgy results, tastes better the next day!