Apple-Hazelnut Rye Cake

Preparation

1. Cake

   Step 1

   Place a rack in middle of oven and preheat to 350°. Lightly coat a 9"-diameter cake (sides should be at least 2") or springform pan with nonstick spray. Line bottom of pan with a parchment paper round; lightly coat with nonstick spray. Toast hazelnuts on a rimmed baking sheet, tossing once, until golden brown, 8–10 minutes. Let cool.

   Step 2

   Meanwhile, whisk all-purpose flour and rye flour in a small bowl to combine.

   Step 3

   Pulse hazelnuts, sugar, baking soda, salt, cinnamon, coriander, and nutmeg in a food processor until mixture looks sandy. Add eggs, dates, and oil and process to a thick paste (mixture will resemble freshly ground nut butter). Transfer to a large bowl and add flour mixture. Using a rubber spatula or wood spoon, mix until almost no dry streaks remain. Add apples and, using wet hands, mix apples into batter (apples will be barely surrounded by batter). Scrape batter into pan; press down firmly and evenly into pan with wet hands (batter will be very thick and sticky).

   Step 4

   Bake cake, rotating halfway through, until very deep golden brown and a tester inserted into the center comes out with only a few moist crumbs attached, 70–80 minutes. Transfer pan to a wire rack and let cake cool in pan. (This cake is very fragile when warm, so it’s important to let it cool completely in the pan before attempting to turn it out.)

   Step 5

   Hold a plate upside down over pan and invert to turn out cake; peel away parchment paper. Chill cake 30 minutes (this will make it easier to frost).

2. buttercream and assembly

   Step 6

   While the cake is chilling, whisk egg whites, sugar, salt, and cream of tartar (if using; it helps stabilize the egg whites, giving you lofty peaks) in the bowl of a stand mixer just to combine. Set over a medium saucepan filled with 1" of gently simmering water (the bottom of the bowl should not be touching the water). Heat, whisking constantly, until sugar is dissolved and an instant-read thermometer registers 185°, 8–10 minutes.

   Step 7

   Fit bowl onto stand mixer fitted with whisk attachment and beat on medium-high speed until meringue is stiff and glossy and bowl feels barely warm to the touch, about 5 minutes. With the motor running, add butter a piece at a time, incorporating fully before adding the next piece, 8–10 minutes. If mixture starts to look chunky or liquidy, just keep beating; it will come back together. Add lemon juice and vanilla extract; beat on high speed until buttercream looks glossy and smooth.

   Step 8

   To frost the cake, make sure bottom of cake is facing up (this will provide a flat surface for easy frosting). Scoop buttercream on top of cake and spread a thick layer over the surface with a small offset spatula or butter knife. Spread a thin film of buttercream over sides. Decorate top with edible flowers and/or toasted hazelnuts as desired.

   Do ahead: Cake can be baked 2 days ahead; tightly wrap and chill, or freeze up to 2 weeks. If frozen, thaw in fridge before frosting. Frosting can be made 5 days ahead; cover and chill. Bring to room temperature, then beat in stand mixer before using. Cake (without decorations) can be assembled 3 days ahead; cover and chill. Bring to room temperature before serving.