Preheat oven to 350 degrees. Spread hazelnuts in a single layer on a rimmed baking sheet; toast, stirring occasionally, until skins begin to split, about 15 minutes. Immediately rub hazelnuts vigorously in a clean kitchen towel to remove skins (as much as will come off easily). Let cool.
In a food processor, pulse hazelnuts and almonds until finely ground. (Be careful not to over process; you don't want the mixture to become a paste.) In a medium bowl, whisk together flour, baking powder, cinnamon, salt, and nut mixture; set aside.
In bowl of an electric mixer fitted with the paddle attachment, beat butter and granulated sugar until light and fluffy, about 3 minutes. Add egg yolk and beat 1 minute to combine. With mixer on low speed, add ground nuts and flour mixture all at once.
Dump the dough into a large zip-top plastic bag and press into a flat disk. Refrigerate for at least 30 minutes and up to 1 day before using.
On a lightly floured work surface, roll out the dough into 12 inches (30 cm) in diameter and about 1/8 inch (3mm) thick. Transfer to a 9 ½ - inch (24cm) tart pan and ease into the tart pan, patting it firmly into the bottom and up the sides. Trim the edge. Refrigerate or freeze the dough until firm, about 30 minutes.
Position the rack in the middle of the oven and preheat to 375 degrees Fahrenheit. Line the tart shell with foil and fill with pie weights (beans for example). Bake until the crust starts to look dry, about 15 minutes. Remove the foil and weights, and continue baking until golden about 15 minutes longer. Let cool completely. Reduce the oven temperature to 350 degrees Fahrenheit. Remove and set aside.
To make the filling/spread
In a bowl, using an electric mixer, beat the butter on medium speed until smooth. Add the almond paste, one piece at a time, beating until smooth after each addition. Continuing to beat, sprinkle in the sugar. Add the egg and mix well. Stir in the flour. Spread the almond paste mixture over the dough. Spread the jam evenly over the almond paste mixture.
Bake until the jam starts to bubble about 20 - 25 minutes. Sprinkle evenly with the sliced almonds. And let cool completely on a wire rack. Remove the pan sides and serve.
Adapted and modified from Martha Bakes and William-Sonoma
Please keep in mind that nutritional information is a rough estimate and can vary greatly based on the products used.