Step 1: Prepare the Almond Cake. Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan. In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Step 2: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla and almond extracts.
Step 3: Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined; do not overmix. Pour the batter into the prepared baking pan and spread evenly.
Step 4: Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cake cool completely in the pan on a wire rack. Once cool, cut the cake into 1-inch cubes.
Step 5: Make the Raspberry Sauce. In a medium saucepan, combine 2 cups of raspberries, 1/4 cup sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the raspberries break down and the mixture is simmering, about 5-7 minutes.
Step 6: Stir in the cornstarch slurry and continue to cook, stirring constantly, for another 1-2 minutes until the sauce thickens. Remove from heat and let it cool completely. You can strain the sauce through a fine-mesh sieve for a smoother texture, or leave it chunky for more texture.
Step 7: Prepare the Almond Mascarpone Cream. In a large mixing bowl, combine the softened mascarpone cheese, cold heavy whipping cream, sifted powdered sugar, almond extract, and vanilla extract.
Step 8: Using an electric mixer, beat on medium-high speed until the mixture is thick, fluffy, and holds stiff peaks. Be careful not to overbeat, as mascarpone can curdle.
Step 9: Assemble the Trifle. In a large, clear trifle bowl (or individual serving dishes), begin by placing about one-third of the almond cake cubes at the bottom.
Step 10: Drizzle about one-third of the cooled raspberry sauce over the cake layer.
Step 11: Spread about one-third of the almond mascarpone cream evenly over the raspberry sauce.
Step 12: Arrange about one-third of the fresh raspberries (from the 3 cups reserved for assembly) over the cream layer.
Step 13: Repeat the layers twice more, ending with a layer of almond mascarpone cream.
Step 14: Garnish the top with the remaining fresh raspberries and toasted slivered almonds, if using, and a few sprigs of fresh mint for visual appeal.
Step 15: Chill the Trifle. Cover the trifle and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to meld and the cake to soften. Serve chilled.