Preheat the oven to 375℉. In the bowl of a stand mixer with the paddle attachment, beat together the butter and sugars until smooth and fluffy. Add the egg and vanilla, then beat again until combined.
Sift the oat flour, tapioca starch, baking soda, baking powder, and salt into a medium mixing bowl. Whisk to combine.
Pour the dry ingredients into the bowl of the stand mixer. Beat the ingredients together on medium until combined.
To flavour the dough, divide it into three even portions. Each portion will weigh about 176 grams. Leave one of the portions of dough in the bowl of the stand mixer. One portion of the dough will be left as is, so this portion can be set aside.
Add the strawberry powder to the dough in the stand mixer. Beat on medium until fully incorporated. Scrape the strawberry dough out of the mixer's bowl into a clean bowl.
Transfer a second portion of dough to the mixer's bowl; don't worry about washing the bowl before. Add the matcha powder and 1 teaspoon of water to the mixer, then beat on medium until flly combined.
Line a small baking tray with parchment. Starting with the plain dough, use a teaspoon to portion the dough into 20 little balls, placing each ball on the lined baking sheet as you roll them. Repeat with the strawberry dough, and lastly the matcha dough. I keep a glass of warm water on the counter to dip the teaspoon into as needed to prevent the dough from sticking.
To form one cookie, take one portion each of plain, strawberry, and matcha dough in your hand. Gently press the dough together and roll into a singular ball of dough. You want the dough to be compact, but each of the flavours should still be visible. See the photos above for a visual. Repeat with the remaining dough.
Organize the dough 2 inches apart on a large lined baking sheet. Bake for 7-8 minutes, rotating the pan halfway.
Allow the cookies to cool for 5 minutes on the baking sheet before transferring to a cooling rack. Repeat the baking process with the remaining dough.
The cookies can be stored in an airtight container at room temperature for up to five days. For longer storage, freeze the cookies for up to two months.