Step 1Line the bottom and sides of a 13x9-inch baking pan with one sheet of parchment paper.
Step 2Blend the flour, hazelnuts, sugar, cinnamon and salt in a mixing bowl. Make a well in the center and cut in the butter in small pieces. Add the yolk and vanilla, then pour the chocolate over all.
Step 3Blend the ingredients together with your fingertips, then when it becomes a cohesive dough, knead gently into a ball.
Step 4With a fork or your fingertips, gently smooth the dough so it evenly covers the paper in the pan. Set the dough in the freezer to chill while you make the hazelnut layer.
Step 1Whisk the whites, sugar and salt together in a bowl until the whites are frothy and the sugar has dissolved, 1 to 2 minutes. Add the hazelnuts and blend with a fork, then blend in the lemon juice.
Step 1Heat the oven to 350 degrees.
Step 2Remove the Chocolate Layer from the refrigerator and spread the jam evenly over the top, using the back of a spoon.
Step 3Spread the Hazelnut Layer evenly over the jam.
Step 4Bake on the lowest rack of the oven for 10 minutes, then bake in the center of the oven until golden brown, 25 to 30 minutes more.
Step 5Cut into bars while hot (if desired, first cut around the pan at the point where the edges start to darken and later discard these parts). Be careful not to cut through the paper. Cut the 13-inch side into 12 sections, then make 4 even cuts the other way. Cool in the pan.
Step 6Using the paper, lift the bars out of the pan, if possible.
Step 7Melt the chocolate and drizzle it in swirls over the top of the bars. After the chocolate has set, remove the bars from the paper and store them in an airtight tin or box. They will keep fresh in a cool place at least 1 week.