Preheat your oven to 350°F (175°C). In a large mixing bowl, combine almond flour and powdered sugar, then cream in unsalted butter until smooth and fluffy, about 2 minutes. Beat in the eggs one at a time, followed by the almond extract, mixing until fully incorporated.

Line a rectangular baking pan with parchment paper and spread the cake batter evenly, smoothing the top with a spatula. Bake in the preheated oven for 25-30 minutes or until the edges are golden and a toothpick inserted in the center comes out clean.

Let the cake cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely.

Once your cake is completely cool, carefully remove it from the rack and place it on a cutting board. Use a sharp knife to cut the cake into petit four-sized squares, about 1-inch in size.

In a medium mixing bowl, combine softened butter and powdered sugar, then beat until light and fluffy. Gradually add milk to achieve desired buttercream consistency.

Melt white chocolate in a microwave-safe bowl in 30-second intervals, stirring until smooth and completely melted. Dip each petit four piece into the white chocolate, ensuring they are fully coated.

While the white chocolate coating is still soft, delicately press almond slices into the top of each petit four.

Let the coated petit fours sit at room temperature until the white chocolate is firm, about 30 minutes. Once set, transfer them to a serving platter.