I Added BTC Staking to a Stock Trading App in 200 Lines of Code

What if your users could earn 3.3% APY on Bitcoin without ever knowing they're using a blockchain? That's what we built with SmoothYield — a traditional finance mobile app (stocks, cash, portfolio) with a hidden superpower: passive BTC yield powered by Starknet. The user signs in with their email (6-digit code) or Google, taps "Stake BTC now", and starts earning. No wallet address. No gas fees. The entire blockchain layer is built with Starkzap , the official Starknet developer toolkit. This post walks through exactly how we integrated it — step by step — so you can do the same in your own app. What the user sees vs. what actually happens From the user's perspective, SmoothYield looks like any fintech app: Sign in with email (OTP code) or Google View stocks (AAPL, TSLA, NVDA) and crypto prices Tap "BTC Yield" and stake for 3.33% APY See "Earning" status on the portfolio screen What the user never sees: a wallet address, a seed phrase, a gas fee prompt, or the word "Starknet". Behind th