For meringue, place egg whites in a large bowl; let stand at room temperature for 30 minutes. Line a baking sheet with parchment.
Beat the egg whites, vanilla, salt and cream of tartar until soft peaks form. Add sugar, 1 tablespoon at a time, beating until stiff glossy peaks form and sugar is dissolved. Fold in nuts.
To form crust, place meringue mixture on parchment; spread in a 14-in. diameter circle (or in several smaller circles for individual servings). Bake at 275° for 1 hour; turn off heat and leave meringue in oven for at least 1 hour longer.
Remove from the oven and cool on baking sheets. When cooled completely, remove meringue from parchment.
For custard, in a large saucepan, combine sugar and flour. Stir in milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to low; cook and stir for 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes. Remove from the heat; gently stir in the butter, vanilla and salt.
Pour into bowl; cover with plastic wrap and cool completely. (Can be made on the day before use.)
Just before serving, beat cream and sweeten to taste. In a microwave, melt chocolate chips and butter; stir until smooth. Spoon custard mixture on meringue; top with whipped cream and fresh berries. Drizzle with chocolate mixture.