At the end of the Pipe Vault, you find the land of Yo'ster Isle, where you can join the Yoshi races.

Explore Yo'ster Isle

You can reach Yo'ster Isle by going through the Pipe Vault.

Talk to the Yoshis

You will find that the Yoshis don't say anything when you talk to them, except for the green Yoshi. Talk to the green Yoshi, then jump on his back to ride around. Then you can talk to the other Yoshis. If you talk to one of the pink Yoshis near the northeast entrance, you can say yes when asked if you can beat Boshi, and you will get some Yoshi Cookies. Then go talk to Boshi to race him.

Race Boshi

During the race, (contrary to what Toad said to do), press B twice, then A twice, and repeat the pattern until the race is over. You can also press Y to eat a Yoshi Cookie to get a burst of speed. You have three cookies.

After you win, you get Yoshi Cookies in your inventory. You can use them on an enemy during battle to summon Yoshi, who will eat the enemy and give you an item. The item that you get will depend on what type of enemy was eaten. If Yoshi couldn't eat the enemy, you get a Yoshi Candy instead.

If you have at least five cookies and you want to store them for later, talk to the pink Yoshi to the left of the goal. If you run out of Yoshi Cookies, talk to the yellow Yoshi to the left of the eggs near the goal.

You can talk to Boshi to race again to try to win more Yoshi Cookies.

When you're ready to move on, go southwest from the racetrack to return to the World Map, and from there go northeast the Pipe Vault, and west to Moleville.