First draw the birthday cake candles on A4 paper as shown below. Draw the flame of the candle in the shape of a droplet. Then just draw a thin strip of diamond rectangle as the body of the candle.

Then draw the outer outline of the top layer of the cake, as shown below. You can just draw this part as a flattened cylinder.

Next, draw the buttercream for this layer of the cake, as shown below. Draw this part with a simple an irregular curve, as the buttercream can be.

Then draw the middle layer of cake using the above method hair as shown below.

Finally, draw the bottom layer of the cake. A birthday cake is drawn, I hope you can enjoy it.