Prep headstand mixer or electric hand mixer. If you are using an electric hand mixer, you will need a bigger bowl to mix all ingredients together.
Start with beating room temperature vegan butter at medium high speed. Once butter has spread with mixture, add brown and white sugar. Mix in medium speed until it reaches its soft peak (or it is looking softer and airy).
Add milk as it continues to mix. Once well combined, add pumpkin purée and mix thoroughly.
Pour vanilla extract and mix. Consistency should be soft and (slightly) loose. However, it should not look runny.