Step 1In a bowl, whisk together the egg yolks, one-fourth cup sugar and the salt. Cook the milk, cream, vanilla bean (if using) and the remaining one-half cup sugar in a heavy saucepan over medium heat, stirring occasionally, until almost simmering. Slowly pour the liquid into the egg-sugar mixture, whisking as you pour. Return the cream-milk mixture to the saucepan. Cook over medium-low heat, stirring constantly with a heat-resistant plastic or wooden spatula, until the custard reaches 175 degrees and lightly coats the spatula.
Step 2Strain the custard into a clean bowl, discarding the vanilla bean, and cool over an ice bath until room temperature. Whisk in the mascarpone. Add the vanilla extract (if using). Refrigerate the custard for at least 4 hours or up to overnight.
Step 3If necessary, whisk the ice cream base to combine. Churn in an ice cream machine according to the manufacturer's instructions. Freeze until scoopable, about 4 hours, depending on your freezer.
Step 1In a large pitcher, combine the lemon juice, sugar and salt. Stir until the sugar is dissolved. Stir in the seltzer water.
Step 2Place some strawberries in 8 (12-ounce) glasses. Fill each with the lemon seltzer water. Add a generous scoop of mascarpone ice cream into each. Serve immediately, with straws and long spoons.