In a bowl, mix together the peanut butter, oats, melted butter and confectioners sugar until fully combined.
Lay parchment paper over a baking sheet, and transfer the peanut butter mixture to the paper. Using your hands, press it down until the peanut butter mixture is about 1/2" thick.
Place the baking sheet flat in the refridgerator for 1 hour, or in the freezer for 15 minutes.
When ready, melt 12 ounces of the white chocolate, saving the rest for the drizzle. You can melt it by placing it in a glass oven safe bowl, and heating in the oven for 15 minutes at 200 degrees. OR you can microwave the white chocolate chips with the microwave set to 'defrost' setting. Make sure to stir frequently to prevent burning. When fully melted, stir in the matcha powder.
When done chilling, remove the baking sheet from the fridge/freezer. Carefully dip the chilled shamrocks in the melted chocolate. You can do this using a tooth pick, or by hand. You also have the option to keep the shamrocks flat on the baking sheet and spoon the melted chocolate over them so they keep their shape, as they can be fragile. When done, chill again.
When ready to decorate, melt the remaining white chocolate, and add 1/4 tsp of any flavor extract you like. Scoop it in a spoon and drizzle over the top. Feel free to add sprinkles, gold dust and anything else that a leprechaun might like!
Keep in the refridgerator in an air tight container until ready to serve! Happy Saint Patrick's Day!