Blockchain Engineer (Smart Contracts) (72K€-90K€)

One of our clients, a fast-growing startup based in Barcelona, is looking for a Blockchain Engineer (Smart Contracts) to take ownership across their onchain product surface: smart contracts, wallet/vault architecture, and the overall developer experience.

They are building developer-first infrastructure, where engineering and product are tightly connected. The team values simple primitives, intuitive APIs, and rapid iteration. This role goes beyond shipping smart contracts — it’s about turning complex wallet infrastructure into something developers truly enjoy using.

What you’ll do

• Design, build, and ship Solidity smart contracts powering wallets, vaults, and account systems (upgradeability, access control, permissions, sessions, spend limits, etc.).
• Own end-to-end product features from spec → contract architecture → implementation → testing → deployment → monitoring, driven by real user needs.
• Build wallet and vault primitives with a strong security mindset, including key management patterns, recovery flows, signing/authorization, modular permissions, and safe upgrade paths.
• Collaborate closely with product and platform teams to design clean, developer-facing interfaces (SDK/API integration points, contract ABIs, standards alignment).
• Enforce smart contract best practices: high code quality, audit readiness, property-based testing, threat modeling, and gas/performance trade-offs.
• Iterate quickly by prototyping, validating with users, and refining solutions without over-engineering.

Your profile

• 4+ years of engineering experience, with strong Solidity smart contract development in production.
• Strong product orientation: you care about UX/DX, trade-offs, and shipping improvements that users can clearly feel.
• Proven experience building secure onchain systems (wallets, vaults, DeFi primitives, authentication/permissioning, or similar).
• Comfortable with modern smart contract workflows: Foundry or Hardhat, unit and invariant testing, CI, deployments, and monitoring.
• Solid understanding of Ethereum architecture, EVM behavior, common attack vectors, and audit-style thinking.
• You are an individual contributor, not a manager.

Nice to have

• Experience with wallet or vault design, including account abstraction patterns, session keys, recovery flows, spend policies, multisigs, and permissions frameworks.
• Familiarity with Rust and/or Solana programs (optional).
• Contributions to open-source developer tooling (contracts, SDKs, libraries, standards).
• Recognition in competitive environments such as CTFs, hackathons, or coding contests.

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