Make the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with liners. Whisk the flour, salt and baking soda in a medium bowl. Combine the cocoa powder and espresso powder in a large bowl. Heat the milk in a small saucepan over medium heat until hot but not simmering; pour over the cocoa powder mixture and whisk until smooth. Let cool a few minutes, then add the granulated sugar and whisk until dissolved. Whisk in the vegetable oil, egg and vanilla, then whisk in the flour mixture until combined.
Divide the batter among the prepared muffin cups, filling each about two-thirds of the way. Bake until the tops of the cupcakes spring back when gently pressed, about 20 minutes. Stir together the strawberry preserves and lemon juice and brush on the warm cupcakes. Transfer to a rack to cool completely.
Meanwhile, make the chocolate-dipped strawberries: Microwave 4 ounces semisweet chocolate in a bowl in 30-second intervals, stirring, until melted. Dip the strawberries in the chocolate; place on a plate. Let set, 20 to 30 minutes.
Make the frosting: Microwave the white chocolate and the remaining 2 ounces semisweet chocolate in separate bowls in 30-second intervals, stirring, until melted. Let cool. Beat 6 tablespoons butter and a pinch of salt in a bowl with a mixer on medium-high speed until smooth. Beat in the melted white chocolate. Add 1 cup confectioners¿ sugar; beat until fluffy, 3 minutes, then beat in 1 teaspoon vanilla and 1 tablespoon milk. Set the white chocolate frosting aside. Repeat this step with the melted semisweet chocolate to make a semisweet chocolate frosting.
Spoon the white chocolate frosting onto one side of a piping bag fitted with a large round tip. Spoon the semisweet chocolate frosting next to the white chocolate frosting so the two flavors are side by side; pipe the swirled frosting onto the cupcakes. Top each with a chocolate-dipped strawberry.