Preheat the oven to 350°F.
Line a sheet pan with a piece of parchment paper or silicon baking mat
Place the butter, brown sugar, and granulated sugar in the bowl of a mixer. Beat until light and fluffy, 1-2 minutes.
Add the egg, peanut butter and vanilla to the bowl and mix until combined
Add the flour and baking soda to the bowl and mix until just combined. Stir in the oats.
Add the M&M's and white chocolate chips to the bowl and gently fold them into the dough.
Make 2 tablespoon sized balls of cookie dough and place them 2 inches apart on the prepared sheet pan. You will need to bake the cookies in batches.
Press the top of each cookie dough ball into the sprinkles.
Press a few M&M's and white chocolate chips onto the top of each cookie dough ball.
Bake for 11-13 minutes or until edges are lightly browned.
Cool the cookies on the pan for 5 minutes, then transfer to a cooling rack
Repeat the process with the remaining cookie dough. Cool all cookies completely then serve.
Store any leftover cookies in a sealed container for up to 5 days, or you can freeze the cookies for up to 2 months.