Toast walnuts in oven or in a pan on stove top.
In oven, take walnuts on a baking sheet, and toast for 8-10 mins. Take out of the oven. let it cool completely.
To roast on skillet, keep on stirring in skillet/pan on stove top on medium flame, till it gives aroma of roasted nuts.
Preheat oven to 180°C or 350°F for 15 mins(preheat timings apply for otg)
In a mixing bowl, cream together butter, sugar.
Add eggs one by one and vanilla essence. Beat and mix well.
Add flour, baking powder, cocoa powder, chopped walnuts, white choco chips and fold lightly.
Use hand to bind the dough. Do not knead the dough. Just lightly bind it.
Transfer to baking silicone sheet or parchment paper then shape in log form.
Then carefully transfer to baking tray.
Lower the temp. of oven to 160°C or 320°F.
Bake in preheated oven at 160°C or 320°F for 30 mins.
Once done, take out of oven and let it cool completely.
Again, preheat oven to 180°C or 350°C.
Cut into thin slices of about ½" . Line on the baking tray as shown in the pic. Do it very carefully, pieces can crumble also.
Then bake at 180°C or 350°C for 30 mins. each from both sides. Takes about an hour(60 mins.)
Once done cool completely and store in an airtight container.
Enjoy with coffee/hot chocolate or masala tea.