Whisk the condensed milk, espresso powder, 1 tablespoon heavy cream and the salt in a large bowl until smooth. (The espresso powder may look speckled.)
Beat the remaining 1 1/2 cups heavy cream in a separate large bowl with a mixer on medium-high speed until stiff peaks form, 1 to 2 minutes. Fold about half of the whipped cream into the condensed milk mixture with a rubber spatula until combined, then fold in the remaining whipped cream until no white streaks remain.
Dot the mixture with the dulce de leche and sprinkle with all but a few teaspoons of the toffee bits; drag a rubber spatula through the mixture to lightly swirl.
Spoon the mixture into a freezer-safe 9-by-5-inch loaf pan or 2-quart baking dish. Dot with more dulce de leche and sprinkle with the remaining toffee bits; drag a rubber spatula through the top to lightly swirl. Cover with plastic wrap and freeze until firm and scoopable, at least 6 hours or overnight.