Today's process for designing, specifying, installing and testing building HVAC control is not digitalized, leading to expensive manual workflows and missed operational performance. To digitalize the design-build-operate process for building HVAC control, the authors developed a process, associated tools and initiated the voluntary ASHRAE Standard 231P. This paper describes this process and tools, which are both based on Modelica. Standardization through a proposed voluntary ASHRAE Standard and the existing Modelica Language Specification provides a robust technology foundation for industry investment. It is based on declarative specification of the control logic, and allows reuse of existing technologies for open-loop and closed-loop control testing through coupling with an HVAC system or a whole building energy model. It supports control testing using MIL, SIL and HIL, and export of digital twins for operational support. The process and ASHRAE Standard 231P have been designed to accommodate existing Building Automation Systems product lines, while also enabling direct code generation such as by using FMI or eFMI. Control deployment can be digital or manual and conformance to the digital specification can be tested formally and programmatically at each step of the control delivery.