Melt the sugar in a small saucepan over medium heat. Stir frequently. This should somewhere in the realm of 5-10 minutes. If it starts to get super clumpy, lower the heat.
Add in the butter and whisk to combine.
Once the butter has been fully incorporated, lower the heat (to medium low, if it's not already there) and slowly pour in the cream with one hand while whisking with the other. It may get pretty bubbly, so be careful!
Add in the miso and vanilla extract and whisk until fully incorporated. Then transfer to an airtight container and let it cool! (Oh, and if your sauce contains a lot of clumps, strain it before letting it cool. Don't feel bad - sugar work is hard!)
Notes
Store in the fridge for up to 2 weeks if not using immediately!