Preheat oven to 350 degrees Fahrenheit and line baking sheets with parchment paper.
Melt butter and let it cool for 5 minutes.
Sift together flour, baking powder, baking soda, and kosher salt in a bowl; set aside.
In a separate bowl, mix melted butter with light brown sugar and white sugar until combined.
Stir in the egg and vanilla extract into the butter and sugar mixture.
Add wet ingredients to the dry ingredients and stir until just combined.
Fold in white chocolate chips and pastel M&Ms.
Scoop dough onto prepared baking sheets using a medium cookie scoop.
Bake cookies for 10 to 12 minutes until golden.
While baking, cut a handful of M&Ms in half to create bunny ears.
Press the M&M halves as ears into the hot cookies near visible M&Ms or add some M&Ms before pressing ears.
Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.