Preheat the oven to 350°F.
In a medium bowl, whisk the oats, flour, baking powder, baking soda, and salt.
In a large bowl or the bowl of a stand mixer, beat the butter, brown sugar, and sugar, and mix until combined. Add the whole egg, egg yolk, and vanilla, and mix until smooth.
Add the dry ingredients to the wet ingredients and mix to form a thick dough. Gently fold in the chocolate chips and Cadbury eggs
Scoop the dough into balls and place on a parchment or silicone-lined baking sheet. Bake 12-14 minutes, until light golden brown on the bottom.
Remove from the oven and let cool on the baking sheet for 5-10 minutes, then move to a wire rack.