Chocolate purple sweet potato roulade

Ingredients:

Recipe adapted from ???? with changes to the filling.
Ingredients for cocoa batter:
25g butter
20g icing sugar
25g egg whites
20g top flour
5g cocoa powder

Ingredients for purple sweet potato filling:
250g purple sweet potatoes (peeled, steamed and mashed)
50g icing sugar
30g butter
50g whipping cream

Ingredients for sponge:
20g butter
60g almond powder
30g icing sugar
120g eggs (about 2)
100g egg whites (about 2.5)
60g fine sugar
50g top flour

Preparation:

Steps for cocoa batter:
1. Soften the butter, mix with all ingredients.
2. Pour on baking tray, use a scraper to smoothen the cocoa batter.
3. Use comb scraper to comb out the line pattern on batter. Place in freezer for 30 mins.

Steps for filling:
1. Add icing sugar and butter to the mashed purple sweet potatoes while it's hot. Use a spatula to stir well.
2. Gradually add the whipping cream into the purple sweet potatoes and mix until it is smooth. Set aside in fridge for 30 mins.

Steps:
1. Melt the butter.
2. Whisk the almond powder, icing sugar with full egg until the batter turned pale.
3. Beat egg whites until frothy, add 1/2 sugar and beat until soft peaks. Put in remaining sugar gradually and beat until stiff.
4. Fold ¼ of egg whites into (2) and then another ¼ egg whites.

5. Pour egg batter into the remaining egg whites and fold (no need to be too thorough).
6. Shift in the flour in two batches, fold well.
7. Pour small amount of the flour batter into (1), mix well and pour the butter batter into the remaining flour batter, fold until well combined.
8. Pour into baking tray and bake at 190C for 12~15 mins.
9. Remove the sponge from tray once the sponge is out from oven. Let it cool completely before apply the filling.
10. Roll up the sponge in swiss-roll style and place in firdge for at least 1 hour before cutting the edge.