1. Combine the lemon zest and juice in a nonreactive saucepan with the sugar and eggs and beat well. Add the butter and cook over medium heat, stirring constantly with a rubber spatula or wooden spoon, until the sugar dissolves and the mixture thickens into curd. Be sure to keep scraping the bottom of the pan during the few minutes this takes; you don’t want the eggs to scramble before the curd forms. If you’re the nervous type, make the curd in a heatproof bowl over a pot of boiling water (the bottom of the bowl shouldn’t touch the water). The curd will just take a little longer to thicken, about 5 minutes. Strain the curd into a bowl and press a piece of plastic wrap against the surface so a skin doesn’t form. Refrigerate until cool, at least 1½ hours.
2. Preheat the oven to 350°F. Grease a 9-inch spring form pan with 1 tablespoon of the butter and dust it with 1 tablespoon of the flour.
3. Cream the remaining 8 tablespoons butter in a large bowl with 1 cup of the sugar until light and fluffy. Sift the remaining 1 cup flour, the baking powder, and salt together and stir into the creamed butter.
4. In a separate bowl, beat the eggs until they start to foam. Do not overbeat, or the cake will be tough. Add the eggs and ground almonds to the flour and butter mixture, mixing well.
5. Scrape the batter into the prepared pan. Drop 8 individual tablespoons of lemon curd evenly around the outside of the cake, leaving a 1-inch border around the edge. Try to place the curd so there is an even amount of cake batter between each dollop of curd. Drop 3 tablespoons curd in the center. Refrigerate any remaining curd for another use (hint: breakfast toast). Sprinkle the top of the cake with the sliced almonds and 1 to 2 tablespoons sugar, according to taste.
6. Bake until the cake is toasty brown on top and a toothpick or knife inserted into the cake comes out clean (be sure not to insert it near the curd), about 40 minutes. Let cool for 10 minutes, then remove the sides of the spring-form pan and let cool completely on a wire rack.
7. If serving the cake with whipped cream, add the almond liqueur to the heavy cream and beat until the cream is stiff. Sift a thin, even layer of confectioners’ sugar over the top of the cake.
8. Present the cake at the table, and offer the almond whipped cream on the side.