Add all crust ingredients to a food processor and pulse until the mixture comes together.
Press crust into a 9" tart pan with a removable bottom and place in freezer while you make the ganache filling.
In a small saucepan, heat heavy cream over medium heat until very hot, but not boiling. Pour over chopped chocolate in a heat-safe bowl and let sit for 2 minutes to allow the chocolate to melt.
After a few minutes, whisk well until completely smooth. If the milk wasn't hot enough and the chocolate isn't completely melted, you can pop in the microwave for 10-20 seconds.
Pour chocolate ganache into crust and return to freezer to set. The filling takes about an hour to set, so wait to start the meringue until it's solid as you cannot top the tart if it's not set.
To make the meringue, set up a double boiler or place a metal bowl over simmering water. Add egg whites, sugar and cream of tartar and use an electric hand mixer to beat at medium speed until the mixture reaches 160 °F (71 °C) on a thermometer (this takes 5+ minutes).
Transfer to a large bowl and continue to beat on high until stiff peaks form (another 5 or so minutes).
Spoon or pipe meringue onto chilled tart. Toast the meringue using a culinary torch, then serve. Store leftovers in refrigerator.
Notes
This s’mores tart can be left out for about 2 hours, but after that, it needs to be refrigerated. It will last for up to 2 days; cover the tart with plastic wrap or transfer any leftovers to an airtight container.