Mix the butter and sugar together until it is light and pale in color
Gradually add in the egg whites
Mix in the vanilla extract
Add in the flour and mix until well combined
Transfer batter into molds
Bake at 170C/338F for 9-10 minutes. Keep an eye on them. Take the cookies out when they turn golden brown around the edges.
Melt the white chocolate then transfer the chocolate into the mold. Place into freezer for about 15 minutes until it sets.
Assemble the cookies by placing some melted chocolate on the cookies then sandwiching a piece of white chocolate between the cookies.