Step 1:
I glued two paper squares of about 15x15 centimeters together. The backside is white.
Fold over these two lines.

Step 6:
Fold in half

Step 5:
Prefold the edges to the prefolded lines.

Step 2:
This is the other side, I used transparent tape to make the chocolate part more shiny.
Prefold over this line.

Step 4:
Prefold both edges to the centerline.

Step 3:
Prefold in half.

Step 10:
Fold this edge into a straight angle too.

Step 8:
Fold over the prefolded line, make the same fold on the other side. These two folds make the model 3d.

Step 9:
Fold this edge into a straight angle, use the prefolded lines as help.
Use glue or tape.

Step 7:
Fold the edge to the inside (Make the same fold on the other side).
Use glue or tape to close the edge.

Step 16:
Fold tiny white papers in half a few times to become sugar crystals.
The sugar crystals also go on top of the cake.

Step 15:
Curve the tip of the whipped cream.
It goes on top of the chocolate cake.

Step 12:
Use a white paper for the whipped cream.
Roll up diagonal.

Step 11:
12 of these cake slices are enough for one complete chocolate cake.

Step 14:
Pull / curve the edges outwards

Step 13:
Fold the flaps inwards.

Ready!
This is a video tutorial for folding the cake slices: