Whisk egg yolks and sugar together in a saucepan until well combined.
In another saucepan, heat whole milk until just simmering. Remove from heat.
Pour milk into egg and sugar mixture slowly, stirring constantly. Return to medium heat until temperature reaches 165 degrees Fahrenheit.
Remove the pan from the heat, then pour into a container and refridgerate for at least 2 hours.
After 2 hours, whisk heavy cream, vanilla, butterscotch syrup, and cream soda in a bowl. Add in the custard base and stir to combine.
Place in your ice cream maker and process according to the instructions. Enjoy!