Line a 9x5-inch loaf pan with plastic wrap, making sure there is enough wrap to hang over the sides.
In a mixer fitted with a whisk attachment, whip cream until soft peaks are formed. Fold-in coffee and vanilla extract.
Place egg whites in the top of a double boiler. Add the sugar and cream of tartar. As egg whites are being heated whisk them with an electric hand held mixer. Continue mixing until the egg whites form soft peaks and the temperature of the mixture reaches 138 degrees F. Remove from heat. Do not over-mix or over-heat the egg whites, or they will become too stiff.
Fold egg white mixture into whipped cream one third at a time. Fold in English toffee bits. Pour mixture into the prepared pan. Spoon the Dark Chocolate Toffee Sauce over the tortoni mixture and then swirl with a blunt knife to combine. (If chocolate sauce is too thick you can heat it for 10-20 seconds in the microwave. Make sure the sauce does not get too hot).
Wrap pan with plastic wrap and place in the freezer overnight or up to three days.
To serve, either remove frozen tortoni from the pan pulling on the plastic wrap lining. Slice and serve with additional Dark Chocolate Toffee Sauce. If preferred, scoop frozen tortoni and serve in small bowls.