In a large bowl, combine the rice flour and brown sugar. Mix well.
Gradually add the coconut milk and water, stirring continuously to create a smooth batter. If using, stir in the crushed peanuts and vanilla extract.
Cut banana leaves or parchment paper into squares. If using banana leaves, lightly heat them over a flame to make them more pliable.
Spoon the batter onto the center of each banana leaf or parchment paper square.
Fold the edges of the leaf or paper over the batter to form a neat package. Secure with string or by folding the edges.
Place the wrapped packages in a steamer basket. Steam over boiling water for about 30-40 minutes, or until the batter is firm and cooked through.
Remove the Koba from the steamer and let cool before unwrapping.
Enjoy your meal!