Combine sugar, flour.
1/2 teaspoon dry mustard, salt, and pepper together in top of a double boiler.
Add 1 cup of the carbonated beverage gradually, blending well.
Cook and stir until mixture comes to boiling; cook 1 to 2 minutes.
Stir in remaining 1/4 cup carbonated beverage.
Vigorously stir about 3 tablespoons of the hot mixture into the beaten egg yolks; immediately blend into mixture in top of double boiler.
Set over simmering water and cook 3 to 5 minutes, stirring occasionally.
Remove from heat and stir in the butter.
Cool; chill thoroughly.