Preheat oven to 350 degrees F.
In saucepan, heat butter and flour, whisking to form a roux. Continue whisking until lightly browned, then pour milk and cream in a steady stream until fully incorporated and thickened. Stir in squash puree, brown sugar, salt, and cinnamon. Transfer to a mixing bowl and whisk in egg yolks.
In a standing mixer, beat egg whites and cream of tartar with whisk attachment until foamy. Move bowl back over double boiler and whisk until mixture is warm, two to three minutes. Return to mixer and beat until meringue forms (soft peaks).
Gently fold meringue into the butternut mixture 1/4 at a time. Pour into six greased individual souffle dishes (or in hollowed butternut bottoms for a double wow effect) and bake until they rise and tops are golden brown, approximately 30-35 minutes.
Garnish with herbs and microgreens.