For the curd, combine zest, juice, sugar and eggs in a heatproof bowl, and beat well. Add butter, and place over a saucepan full of simmering water. Cook, stirring constantly with a rubber spatula until mixture thickens into curd, about 5 minutes. Strain into a bowl, and press plastic wrap onto surface to keep skin from forming. Refrigerate until cool, at least 1 1/2 hours.

Heat oven to 350 degrees. Grease a 9-inch spring-form pan with 1 tablespoon butter, and dust with 1 tablespoon flour, shaking out excess.

With an electric mixer, cream the remaining butter and 1 cup sugar together until light and fluffy. Sift together the remaining flour, baking powder and salt, and stir in. In a separate bowl, whisk eggs until they start to foam. Do not overbeat or the cake will be tough. Add eggs and ground almonds to batter, and mix well.

Scrape batter into the prepared pan. Drop 8 tablespoons lemon curd around perimeter of batter, leaving a 1-inch border. Drop 3 to 4 tablespoons curd into center of batter. Refrigerate remaining curd for another use. Sprinkle cake with toasted almonds and 1 to 2 tablespoons sugar.

Bake until cake is toasty brown on top and a toothpick inserted into cake (not curd) comes out clean, about 40 minutes. Let cool on rack 10 minutes, then remove sides of pan, and cool completely.

Whip cream with almond liqueur. Present cake at table, and offer whipped cream on the side.