Heat oven to 340°F/170°C with convection (or 350°F/175°C conventional). Line a baking sheet with parchment paper.
In a bowl, beat butter and powdered sugar until smooth and creamy. Add egg and mix until fully incorporated. Add almond flour and cornstarch and mix just until a soft, slightly sticky dough forms.
Divide dough in half. Place one portion between two sheets of parchment and roll into an even 8-by-10-inch rectangle about ⅛ inch thick. If dough is too soft to handle, refrigerate 20 to 30 minutes before rolling.
Remove top parchment. Spread a thin, even layer of Nutella or date spread over the dough, leaving a ½-inch border. Sprinkle with nuts and cinnamon, if using. Using the parchment to guide you, roll tightly into a log. Wrap and freeze until firm, about 30 minutes. Repeat with remaining dough.
Place frozen logs on a prepared baking sheet. Bake until lightly golden, 18 to 22 minutes. Cool completely. For clean slices, refrigerate until fully chilled, then cut into even pieces. Dust with powdered sugar and serve.