Build.
Hands-on full-stack — TypeScript/Node, React, Flutter. Before that: years of Laravel and Vue in production. Self-taught since age seven - your stack won't be the problem.
Mattias Siø Fjellvang
I build software that feels finished.
Senior full-stack developer and architect in Copenhagen. Fifteen years shipping products - most visibly Doubble, a dating app I architected and built from 15,000 to 300,000+ users across six markets, with an App Store rating above Tinder, Hinge and Bumble.
Available for selected freelance and retainer work
What I actually do
Finish.
The layer between design and code: the interaction that should run at 60fps, the touch target that's too small, the empty state nobody designed. This is where products are won - and where most teams run out of budget.
Bridge.
Six companies as CTO or co-founder, one of them through to acquisition. I've sat on your side of the table - budget, runway, and "should we build this at all" are part of the engineering conversation, not interruptions to it.
How I work
I work hourly or on retainer, embedded in existing teams. I support the roadmap - I don't own it; I've run enough of my own to mean that. Copenhagen-based, hybrid. Hand me an under-specified problem and you get options back, not questions. I teach as I go - code review, pairing, the reasoning behind the calls.
I write code that doesn't break when I leave — and teams that don't need me to stay.
Selected work
Kite
Ad-performance monitoring · via KlientBoost
Lead architect & backend engineer from day one — multi-tenant platform on CockroachDB with granular RBAC (Casbin), automated alerting across Google Ads, Meta and LinkedIn, and immutable audit trails laid out for SOC 2.
Doubble
Social dating platform
Architected and built the backend and app from 15,000 to 300,000+ users across six markets — 1B+ swipes, 500,000 matches a day on auto-scaling, multi-region infrastructure across the US and Europe. Out-rated Tinder, Hinge and Bumble on the App Store: 4.5/5 across 5,000+ ratings.
The Jewellery Room
E-commerce (WooCommerce → headless)
Led the headless transition: Nuxt/Vue frontend, fully static shop deployed to Cloudflare's edge, automated deploys and testing. Faster site, easier editorial, better UX, more sales.
Wash World
Car-wash subscription brand
Built their international site using a component-based page builder in WordPress with automated backend integrations. Layered dynamic Cloudflare edge caching with instant purge-on-save, leaving marketing and IT 100% self-sufficient.
Athliit
Sports-club SaaS - acquired
Co-founder & CTO. Built the platform and grew the company from 2 to 16 people and 2 to 50 clients before its acquisition by Sportyfriends.
Responsible for building most of the technology at Doubble from the ground up… integrity, grit, speed, and above all an understanding of what really matters.
Always seeking to improve even the smallest of details.
If it wasn't for Mattias, Doubble wouldn't be where it is today.
Picks up on new technologies at a pace I have never seen before.
Started coding as a child - a completely natural approach to technology. An utterly indispensable sparring partner.
A pure joy to work with a backend developer that almost solves my backend problems before I even mention them
The full list - going back to 2009 - is on LinkedIn.