Custard ice cream and soft serve ice cream are both made with dairy as the main ingredient. The thickening agent and preparation method are the main variable between the two types of ice cream. They can both be flavored with any flavor you choose, such as fresh fruits or chocolate, but they both call for vanilla as the base flavor.
Custard ice cream is made with egg yolks as the thickening agent in milk while soft serve uses cream, and sometimes starch, as a thickener. Vanilla custard ice cream usually has a yellow hue from the egg yolks, while vanilla soft serve is usually white.
Because custard ice cream uses egg yolks as the thickener, it must be cooked before it can be chilled and frozen into ice cream. The egg yolks must be tempered with hot milk by slowly adding the milk while whisking so the yolks don't curdle in the process. The custard mixture should be cooled before it's added to an ice cream maker. Soft serve ice cream uses cream as the main ingredient, mixed with pudding mix and sugar. When instant pudding is used, no cooking or cooling is required.
The pudding mix used in soft serve has starches in it that create a smooth, thick consistency after it is churned. When the starch is allowed to bloom completely, the pudding cream mixture becomes thick, creamy and whipped in texture. Because cream is used instead of milk, soft serve feels creamier in your mouth. Custard ice cream is thick and feels rich in your mouth, like custard, because of the egg yolks.
Custard ice cream is slightly richer-tasting than soft serve ice cream as an effect of the egg yolks. Custard ice cream is similar to the flavor of creme brulee because the ingredients are the same, but they are prepared differently. Soft serve tastes plainer than custard ice cream because it is only cream, sugar and vanilla flavoring. Soft serve ice cream tastes similar to sweetened milk or whipped cream.