In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in extracts.

In a medium bowl, whisk together flour, baking powder, and salt. In a small bowl, whisk together milk and sour cream. With mixer on low speed, gradually add flour mixture to butter mixture alternately with milk mixture, beginning and ending with flour mixture, beating just until combined after each addition. Spread batter into prepared pan.

Bake until a wooden pick inserted in center comes out clean, 27 to 30 minutes. Let cool in pan for 10 minutes. Invert cake onto a wire rack, and discard parchment. Let cool completely.

Using a long serrated knife, trim cooled cake on all sides to create a 15x10-inch rectangle. Cut cake in half to create 2 (10x7½-inch) layers.

Spread half of Almond Buttercream (about 3⁄4 cup or about 170 grams) on top of 1 cake layer. Top with remaining cake layer, lightly pressing to adhere. Spread remaining buttercream on second layer, smoothing as much as possible. Freeze for 1 hour. (Alternatively, tightly wrap assembled cake in plastic wrap, and freeze for up to 2 months. Unwrap cake, and let it thaw in the refrigerator overnight before proceeding.)

Line a rimmed baking sheet with plastic wrap; place a wire rack on top. Place no more than 12 petit fours at least 1 inch apart on prepared rack. Carefully pour White Chocolate Poured Fondant onto petit fours, making sure all sides are completely covered, reheating and re-pouring fondant as necessary. (See Note.) Let stand until set, 10 to 15 minutes. Remove wire rack, spoon used fondant into liquid-measuring cup, and reheat as needed in the top of a double boiler or microwave to maintain a temperature range of 105°F (41°C) to 110°F (43°C).

Using a small offset spatula, move glazed petit fours to a serving platter. Repeat procedure with remaining petit fours and remaining White Chocolate Poured Fondant, only removing cakes from freezer when ready to pour. Garnish with gold leaf or melted white chocolate, if desired. Store in an airtight container for up to 2 days, or refrigerate in an airtight container for up to 1 week.