Step 1

Heat the oven to 275°F with a rack in the middle position. Coat the bottom and sides of a 9-inch springform pan with butter. Line the bottom of the pan with kitchen parchment, then butter the parchment. Set a wire rack in a rimmed baking sheet. In a large bowl, combine the bittersweet and semi-sweet chocolates and the butter; set a fine mesh strainer across the bowl, then set aside.

Step 2

Using a vegetable peeler, remove just the outer zest of the orange, not the white pith just beneath, in long strips; set the strips aside. Halve the orange and juice into a liquid measuring cup. Measure 3 tablespoons of the juice into a medium saucepan. Add the bourbon to the remaining juice in the measuring cup, then add enough water to equal 1 cup total liquid; set aside.

Step 3

Add the 160 grams (¾ cup) sugar to the juice in the saucepan, then add the zest strips and peppercorns. Set over medium-high and cook, without stirring but occasionally swirling the pan, until the sugar dissolves, 1 to 2 minutes. Continue cooking, swirling the pan often, until the sugar caramelizes to deep mahogany brown and the peppercorns begin to pop, 4 to 5 minutes. Remove the pan from the heat and carefully pour in the orange juice–bourbon mixture; the caramel will bubble up and harden. Set the pan over medium, bring to a simmer and cook, stirring, until the caramel has dissolved and the peppercorns no longer stick together, 1 to 2 minutes. Remove from the heat and stir in the bitters.

Step 4

Immediately pour the hot sugar syrup through the strainer into the chocolate-butter mixture; reserve the strained solids. Jostle the bowl to ensure the chocolate and butter are fully covered with syrup, then let stand for 2 to 3 minutes. Using a silicone spatula, gently stir until the mixture is well combined and completely smooth; it should be barely warm.

Step 5

Add the beaten eggs to the chocolate mixture and stir with the spatula until homogenous and glossy, 2 to 3 minutes. Pour into the prepared springform pan. Gently tap the sides of the pan to remove any air bubbles, then use the back of a spoon to smooth the surface. Set the pan on the prepared baking sheet and bake until the cake barely jiggles when the pan is gently shaken, about 45 minutes.

Step 6

Remove from the oven and immediately run a thin, sharp knife around the edges to loosen the sides of the cake from the pan. Cool to room temperature in the pan, then refrigerate uncovered for at least 4 hours or up to 24 hours.

Step 7

While the cake cools transfer the zest strips from the strainer to a small, shallow bowl, removing and discarding any peppercorns stuck to them. Sprinkle the strips with the 70 grams (⅓ cup) sugar, then toss until the strips are completely coated. Cover loosely and store at room temperature until ready to serve.

Step 8

About 2 hours before serving, remove the cake from the refrigerator. Remove the zest strips from the sugar and shake off excess sugar; reserve the sugar for another use. Cut the strips lengthwise into thin strips. Remove the sides of the springform pan. Arrange the zest slivers on the cake around the edges. Slice the cake and serve with small spoonfuls of yogurt.