Preparation

Make the crust

• Position a rack in the center of the oven and heat to 325°F. Butter and line a 9×13-inch metal pan with parchment so that the parchment overhangs the short ends by a couple of inches.
• In the bowl of a stand mixer fitted with the paddle attachment, combine the two flours, sugar, and salt. Add the butter and mix on low speed until combined and the mixture looks like coarse sand. Scrape the mixture into the prepared pan and press into the bottom in an even layer. Bake until the crust is dry and slightly puffy, and smells nutty, about 12 minutes. Cool completely on a wire rack.

Make the filling

• In the bowl of a stand mixture fitted with the paddle attachment, beat the cream cheese on medium speed until light, fluffy, and smooth, 4 to 5 minutes. Scrape down the bowl as needed, making sure that the cream cheese is smooth and lump-free. Add the sugar and beat on medium speed until incorporated, stopping to scrape down the bowl, 2 to 3 minutes. Add the cream, vanilla, and salt, and mix on low until combined. Add the white chocolate and butter, and mix on low until combined. With the mixer on low, add the eggs one at a time and then the yolk, beating after each addition. Using a spatula, give the filling a couple of stirs to make sure it’s fully mixed.
• Pour the batter into the cooled crust, then smooth the top, if needed. Gently bang the pan on the counter several times to help get rid of any air bubbles. Bake until the cheesecake is set (the middle will register 150°F on an instant-read thermometer), 20 to 25 minutes. Turn off the heat, open the oven door just a crack, and let the cheesecake cool in the oven for 30 minutes.
• Transfer the pan to a wire rack and let cool to room temperature. Cover and refrigerate at least 4 hours or overnight.

Make the ganache

• Put the chocolate in a small bowl. Heat the cream in a small saucepan until just about to boil. Pour over the chocolate, and let sit for 5 minutes. Add the salt, then whisk until smooth. Cool to room temperature.

Assemble the bars

• Top the chilled cheesecake with the ganache. Sprinkle with the chopped hazelnuts, then refrigerate until set. Use the parchment sling to remove the cheesecake from the pan, sprinkle with the sea salt, if using, then slice into bars while cold. For best flavor, serve at room temperature.