Lightly coat a 3 1/2- or 4-quart slow cooker with cooking spray. In a medium bowl stir together the next four ingredients (through the 1/4 cup water) until nearly smooth. Stir in 1/2 cup hazelnuts. Spread batter in bottom of cooker. Drop chocolate-hazelnut spread by spoonfuls on top of batter. Using a knife, swirl the spread into batter.
In another bowl combine sugar and cocoa powder. Gradually stir in the boiling water and liqueur, if using. Pour evenly over batter in cooker. Cover and cook on high for 2 1/2 hours, giving crockery liner a half turn halfway through cooking, if possible (center may appear moist but will set up upon standing). Turn off cooker. If possible remove liner from cooker. Let stand, uncovered, for 30 to 45 minutes to cool slightly before serving.
To serve, spoon warm cake into dessert dishes; drizzle with the warm hazelnut spread. If desired, sprinkle with additional chopped hazelnuts.