Caramel flatters creamy custard in these bake-ahead treats. Making caramel is easy: Stir together a sugar syrup; simmer until golden; and stir in half-and-half.
Preheat oven to 325 degrees. Set a kettle of water to boil.
Make caramel: In a medium saucepan, combine 1/2 cup sugar and 1 cup water. Cook over medium-high, stirring, until sugar has dissolved. Continue to cook, without stirring, until syrup turns golden brown, 8 to 10 minutes. Carefully add half-and-half (mixture may clump and sputter); simmer, stirring constantly, until smooth. Remove from heat.
In a medium bowl, whisk together egg yolks, remaining 1/2 cup sugar, vanilla, and salt until pale yellow and thick. Stirring constantly with a wooden spoon, gradually add caramel. Skim any foam from surface of custard mixture.
Divide mixture among six 6-ounce ramekins or custard cups. Line a baking pan (just large enough to hold ramekins) with a dish towel; place ramekins on dish towel. Transfer to oven, and pour enough boiling water into pan to come halfway up sides of ramekins; cover pan loosely with foil.
Bake custards just until set but slightly wobbly, 40 to 45 minutes. Carefully remove ramekins from pan. Refrigerate until chilled, at least 4 hours (or, covered, up to 3 days). Garnish with toffee before serving.