Preheat oven to 350 degrees F. Butter a 9-inch round spring form pan and coat with granulated sugar. Line pan with a parchment round.
Melt butter and chocolate together in a bowl over a bain-marie. Once melted, remove from heat and mix in whiskey. Set aside.
In the bowl of stand mixer with a whisk attachment, whisk egg yolks and granulated sugar until pale and thick.
Whisk together ground hazelnuts, salt, cinnamon, and espresso powder. Mix into melted chocolate mixture. Fold thickened yolks into the chocolate mixture.
In a clean bowl, whisk egg whites to soft peaks. Do not over beat. Add whites to chocolate batter in three additions, folding in whites gently.
Pour chocolate batter into prepared pan. Level with a mini offset spatula. Place pears cut side down onto the batter. Bake for 35-45 minutes until pears are soft and cake is cooked throughout. Let cool in pan for 10-15 minutes before unmolding. Serve warm or at room temperature. Sprinkle with powdered sugar or serve with ice cream or creme fraiche.