Separate the four eggs into four yolks and four egg whites in separate bowls. The whites should be in a slightly larger bowl as this is where you’ll make the meringue.
In a heatsafe bowl, add the chocolate (170g) and sliced butter (76g or ⅓ cup). Warm the bowl in the microwave for 60 seconds then stir, if the chocolate is not melting after a few minutes of stirring, set the bowl back in the microwave for another 20 seconds. Stir until smooth, set aside to cool. Alternatively you can do this over a bain marie; a pot of simmering water on the stove.
Use a hand mixer to beat the four egg whites until frothy, you want to be able to look at the bottom of the bowl and not see any liquid - it should all be frothy. This can take 3-4 minutes. Once frothy, slowly pour in the sugar (133g ⅔ cup) while beating, stopping to allow the egg whites to absorb the sugar before adding more. Once all the sugar is in, beat until you have medium peaks: the meringue will be shiny but not hold stiff peaks, there will be a slight droop (see video for guidance).
In the bowl with the egg yolks, whisk in the tablespoon of sugar, salt and vanilla. Pour this mixture into the cooled chocolate butter and stir to just combine.
Use a large rubber spatula for this next part: we’re going to slowly fold the chocolate mixture into the meringue. Pour a bit of the chocolate and use the spatula to make J shapes, folding without stirring - we don’t want to let out the air that is suspended in the meringue. Keep folding until the mix is even, it will be light colored and airy.
When the pie crust has completed its partial blind bake, pour the chocolate filling into the pie crust. Gently smooth the filling until it’s all at an even level. Set the pie in the oven to bake until the filling has puffed up considerably, cracked and does not wobble in the center (it may wobble at the edge cracks, this is fine), about 35 minutes.
Set the pie on the counter, as it cools it will fall. Serve warm or at room temperature. This does't have a gooey filling like most pies so serve it with whipped cream or ice cream.