Preheat the oven to 350 degrees F. Butter just the sides of a 9-inch round cake pan.
For the caramel topping: Microwave the butter, brown sugar and lemon juice in a small microwave-safe bowl until melted. Spread the mixture over the bottom of the prepared pan. Arrange the carrot ribbons on top.
For the cake: Spread the pecans out on a baking sheet, and bake until nicely toasted, 8 to 10 minutes. Let cool, then pulse in a food processor until finely ground. Transfer to a large bowl and whisk together with the flour, cinnamon, baking powder, baking soda, salt and allspice; set aside.
Beat the butter and brown sugar with an electric mixer on medium-high speed in a large bowl until light and fluffy, about 4 minutes. Beat in the lemon zest. Add the eggs 1 at a time, beating to incorporate after each addition and scraping down the side of the bowl as needed (it's okay if the batter looks slightly separated and broken). Reduce the mixer speed to low, and add 1/2 the flour mixture, then the sour cream, then the remaining flour.
Pour the batter over the carrots in the cake pan, spread it into an even layer and give the pan a few taps on the counter. Bake the cake until it's golden brown and pulls away from the edge and a toothpick or cake tester comes out clean when inserted in the center, 45 to 50 minutes. Run a knife around the edge of the cake to separate it from the pan. Let the cake cool in the pan on a wire rack until just cool enough to handle, about 30 minutes. Invert a serving platter over the cake pan and, while holding them together, quickly flip them over so the cake unmolds, carrot-side up. Let cool completely. Cut into wedges, and serve.
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