Backend Engineer

About the role

Cornerstone is looking for a Backend Engineer to help build secure and reliable fintech systems. You will work on real backend applications that power digital savings, merchant payments, collections, transaction processing and financial integrations.

Key responsibilities

• Develop and maintain RESTful APIs using Node.js and Express.
• Design data models and queries with MongoDB and Mongoose.
• Implement backend features for savings, merchants, payments, reporting and account management.
• Integrate third‑party PSP and banking APIs under supervision.
• Handle payment callbacks and webhooks with validation, idempotency and error recovery.
• Write logging, error handling, automated tests and secure authentication.
• Support background job processing with Redis and BullMQ where applicable.
• Collaborate via GitHub pull requests and code reviews.

Required profile

• Working knowledge of JavaScript and Node.js.
• Understanding of Express, REST APIs and asynchronous programming.
• Basic familiarity with MongoDB or another database system.
• Experience with Git and GitHub.
• Knowledge of authentication, input validation and API security.
• Evidence of personal, academic, freelance or professional backend projects.
• Willingness to document and test code.

Required skills

• JavaScript
• Node.js
• Express
• MongoDB
• Mongoose
• Git
• GitHub
• TypeScript
• Redis
• BullMQ
• Docker
• JWT
• Webhook integrations

What we offer

• Code reviews and guidance on building secure, reliable payment systems.
• Flexible, remote‑first working arrangement.
• Practical experience contributing to a developing fintech company.
• Real products and real business challenges.
• Task guidance, feedback and professional development.
• Exposure to cross‑functional collaboration and startup operations.
• Certificate or reference letter on successful completion of the internship.
• Consideration for expanded responsibilities or future roles based on performance.