Line a round baking pan with a baking sheet and keep aside.
Beat the butter and sugar together until the mixture turns fluffy and creamy. Add the flour and mix until it ressembles like a coarse crumbs.
Add in an egg one at a time until they gets well mixed. Add the baking powder to the milk and pour this mixture to the egg mixture and beat until smooth.
Preheat the oven to 350F.
Now mix the lemon juice to the baking soda (it will be fizzy) add this to the cake batter.
Pour the cake batter to the prepared baking pan. Add a coin in the batter.
Meanwhile, mix the sugar and walnuts in a bowl, keep aside.
Bake the cake to the preheated oven for 10minutes.
Remove the cake from the oven, top it with the sugar and walnut mixture. Return to the oven and continue to bake for 20-22minutes.
Put off the oven and cool completely.
Remove the cake from pan and transfer to a serving plate.