Combine the melted butter, sugar, eggs and vanilla. In a separate bowl combine the flour, baking powder and salt. Add the dry ingredients to the wet mixture then stir well until combined. The dough will be thick.
Add the walnuts and raisins. Stir well to combine. Divide mixture into three and form into logs about 10cm wide. Place on a greased or lined baking tray. Brush with egg white.
Bake in the oven for 20 minutes until golden brown. Remove from the oven and allow to cool for about 10 minutes.
Slice each log into 2cm slices. Lay the biscotti on the baking tray and place back in the oven for 10 more minutes, or until golden brown. Cool thoroughly before serving.