In medium saucepan over low heat, melt the butter.
Remove from heat and scatter 1 cup of white chocolate chips over the butter. Do not stir.
Allow to sit for 2 minutes, then gently stir butter and chips together until combined. Set aside to cool to lukewarm (about 10 minutes).
In a large bowl, whisk together flour, brown sugar, baking powder, and salt.
Add the eggs, vanilla, and white chocolate mixture to the flour mixture. Mix until just combined.
Fold in the remaining 1 cup of white chocolate chips.
Fold in the Lucky Charms marshmallows.
Spread into a greased 9” x 13” pan.
Bake at 325 degrees for 20-22 minutes or until edges are golden brown and a toothpick inserted in center comes out clean.