Melt butter in a small saucepan and warm over low heat until it begins to turn golden brown and smell toasted.
Remove the pan from the heat and let the butter cool for five to ten minutes, then stir in the erythritol.
Transfer the butter and erythritol to a blender and add the Keto Chow, heavy cream, water, and rum extract.
Blend until all the Keto Chow is dissolved, and the mixture is creamy.
Chill the mixture for at least one hour.
Mix one cup of the cream soda into the Keto Chow mixture and stir until well blended.
Slowly pour the remaining diet cream soda into the mixture, stirring gently.
Divide the Butterbeer between two well-chilled mugs or glasses and enjoy!