Preheat oven to 350 degrees F.
Line a baking sheet or two with parchment paper.
In a large mixing bowl, add cubed cold butter and sugar. Beat with an electric mixer on medium to medium-high until combined and creamy.
Add egg and vanilla. Beat again until combined by not overmixing.
Add flour, salt, baking powder and cocoa powder and beat together on low speed just until a chocolate dough comes together.
Fold in the white chocolate chips.
Using a medium cookie scoop, (1 & 1/2 tablespoons), scoop cookie dough onto baking sheet and press some white chocolate chips on top, but don't add more white chocolate chips until they are done baking. This helps the white chocolate chips not to brown on top.
Leave room for the cookies to spread very slightly. If dough is warm, refrigerate for 30 minutes or so.
Bake for approximately 8-10 minutes. Gooier for 8 minutes.
Cookies should be starting to firm up but still soft in the middle. Remove from the oven and press a few white chocolate chips on top if desired. Don't do this before baking, as it will make the chocolate chips caramel colored.
Let cookies set on cookie tray to cool before removing them to cool completely on a wire rack. So good with a glass of milk!
Allow to cool completely prior to storing.