MAKE PEANUT BUTTER CHEESECAKE LAYER: Move oven rack to middle position and preheat oven to 325 degrees F. Grease bottom of 9-inch springform pan with non-stick cooking spray, line bottom of pan with parchment paper, and then spray parchment and rest of pan with non-stick cooking spray; set aside.
In the bowl of a stand mixer fitted with paddle attachment, add cream cheese, peanut butter, and sugar and beat on medium speed until combined, about 1 minute. Add flour and beat on low until combined, about 30 seconds. Add egg and milk, and pulse mixer until mixture comes together in a smooth, cohesive batter, about 1 minute.
Pour batter into prepared pan, and use rubber spatula to smooth batter evenly. Bake for 10 minutes, then reduce heat to 260 degrees F and back for 50 minutes, or until center jiggles slightly. Move pan to wire rack and cool completely, about 2 hours. Cover pan with foil, and place in freezer until frozen-solid, about 3 hours or overnight (Note: cheesecake can stay in freezer for up to 3 days).
MAKE BUCKEYES: Line cookie sheet with wax paper; set aside.
In bowl of stand mixer, add peanut butter, butter, vanilla, and powdered sugar. Mix on low until a smooth, stiff dough forms. Shape batter into balls using 2 teaspoons of dough for each ball, and insert a wooden toothpick into each ball. Place each ball on prepared cookie sheet, and chill for 1 hour (up to overnight).
In small bowl add chocolate and vegetable shortening. Cook in microwave for 60 seconds on 50% power. Stir, and cook another 30 seconds on 50% power. Repeat until chocolate is smooth.
Take peanut butter balls out of refrigerator. Dip each into melted chocolate, leaving a small area on top uncovered. Place dipped balls on wax paper-lined cookie sheet, chocolate side down. Chill for 30-60 minutes, or until set. Remove toothpicks, and place buckeyes in air-tight container in the refrigerator until ready to use.
MAKE BROWNIE LAYERS: Move oven rack to middle position, and preheat to 350 degrees F. Lightly grease the bottoms of two 9-inch round cake pans, line pan with parchment rounds, and spray parchment and sides of pan with nonstick cooking spray. Set aside.
In medium bowl, whisk together oil, sugar, vanilla, and eggs.
In large bowl, whisk together flour, cocoa powder, and salt. Slowly pour oil mixture over flour mixture, and mix until combined and no flour streaks remain.
Evenly distribute batter between prepared cake pans, and bake on center rack for 40-45 minutes, or until toothpick inserted comes out with a few moist crumbs.
Cool brownies for 10 minutes in their pans, and then remove them from pans. Cool completely on wire rack, about 2 hours.
MAKE PEANUT BUTTER FROSTING: In the bowl of your stand mixer fitted with paddle attachment, cream butter and peanut butter together on medium-high speed for 2-3 minutes, or until light, creamy, and lightened a few shades.
Scrape down sides of bowl, add powdered sugar, and mix until combined. Add heavy cream and beat on medium-low until combined and smooth. Turn mixer to high and beat for 4 minutes, or until frosting is light and fluffy.
MAKE CHOCOLATE GANACHE: Pour chocolate chips into medium bowl; set aside. In small saucepan over medium heat, heat cream until it almost starts to simmer. Remove from heat, and pour cream over chocolate. Let mixture sit for 5 minutes, and then stir until chocolate is melted and smooth. Stir in corn syrup.
ASSEMBLE CAKE: Remove cheesecake from freezer, and then remove cheesecake from pan (slide knife under parchment to remove cheesecake from bottom of pan). Peel off parchment. Measure cheesecake layer against brownie layers, and trim cheesecake layer as needed so all layers are the same size in diameter. Place one brownie layer on center of cake plate, and place cheesecake layer on top. Place 2nd brownie layer on top of cheesecake.
Reserve ½ cup of peanut butter frosting for later. Apply a crumb coat layer of frosting to cake—use a long, thin spatula to cover the cake completely with a thin layer of frosting. Don’t worry if there are visible crumbs on the frosted cake, because it will get a second layer of frosting.
Refrigerate for about 30 minutes so crumb layer will “set”.
Apply second layer of frosting using a long, thin spatula again to spread frosting evenly over top and down the sides of cake. Refrigerate another 30 minutes to set frosting.
Drizzle chocolate ganache over top of cake, allowing small drips to drip down sides of cake. Smooth top of cake as desired with offset spatula. Refrigerate 30 minutes to set ganache.
Use reserved frosting to fill piping bag fitted with a large star tip (I used a Wilton 1M tip). Pipe 8 swirls of frosting evenly spaced around edge of cake. Place a buckeye on top of each swirl. Place cake in refrigerator. Serve at room temperature.