To make the crust, melt the 1/2 stick of butter and combine it with
cracker crumbs, stirring together until uniform in texture.
Lightly grease a 9-inch springform pan and gently press the crumb
mixture into it, going up the sides by about 1 inch. Scatter the
marshmallows evenly over the crust and set aside.
Over the lowest possible heat, melt together the remaining butter
and both chocolates. Stir occasionally until completely melted and
smooth (about 20 minutes).
Set the oven rack at the upper middle level and preheat the oven
to 375 degrees F.
Stir the vanilla into the cream and add to the chocolate mixture,
beating at medium speed until thoroughly combined. Beat in the eggs,
one at a time.
Stir together the flour, sugar and salt. At medium speed, beat this
into the chocolate mixture in two or three batches to allow it to
be thoroughly absorbed. Scrape down the sides of the bowl and beat
Carefully pour filling over crust, making sure to completely cover
marshmallows (you may need to smooth the top of the filling with
Bake on the upper middle rack at 375 degrees F for 12
to 15 minutes - until the outer edge of the tart looks set but the
inside still seems wobbly.
Remove from heat and allow to cool to room temperature before covering
loosely with plastic and transferring to the fridge.
Allow to chill
for at least 6 hours before cutting and serving.
Serves 12 to 16.
This keeps in the refrigerator,
loosely covered, for several days. It is best served chilled.