In a nonstick pan, combine grated khoya and sugar. Cook on medium flame, stirring continuously, until the sugar dissolves and the mixture starts to leave the pan. This takes about 6~8 minutes.
Let the mixture cool a little bit, then add rose essence and mix well.
Divide the mixture into 2 equal halves. Add cocoa powder in 1 half and mix well to combine. Add pistachios in the remaining khoya mixture and mix well.
Now divide the cocoa mixture into 3 equal parts and roll each part into 8"x3" rectangle.
Divide the pistachio mixture into 3 equal parts and form a roll of 1" diameter and 8" long.
Place the pistachio roll on the chocolate rectangle and roll it up in such a way that the chocolate part covers the pistachio portion evenly from all sides and there are no cracks on the surface. Wrap the roll with plastic wrap and refrigerate for at least 15 minutes.
Repeat with the remaining chocolate and pistachio mixture.
Once the roll is chilled and firm, slice into 1/2" slices.