Preheat oven to 350°F. Beat butter with a heavy-duty stand mixer on medium speed until creamy. Gradually add 1 cup of the granulated sugar, and beat until light and fluffy, 3 to 5 minutes. Add egg yolks, 1 at a time, beating just until blended after each addition.

Whisk together flour, baking powder, and salt in a medium bowl. Add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat on low speed just until blended after each addition. Beat in 1 teaspoon of the vanilla extract.

Beat egg whites with an electric mixer on high speed until stiff peaks form. Fold about one-third of the egg whites into batter; fold in remaining egg whites in 2 batches. Spoon batter into 1 greased (with butter) and floured 9-inch square baking pan.

Bake in preheated oven until a wooden pick inserted in center comes out clean, 25 to 28 minutes. Cool in pan on a wire rack 10 minutes. Remove cake from pan, and cool completely on wire rack, about 1 hour.

Stir together peaches and remaining 1/2 cup granulated sugar in a large bowl. Let stand, stirring occasionally, 30 minutes. Stir in mint.

Gently whisk together mascarpone cheese, powdered sugar, and remaining 2 teaspoons vanilla in a large bowl. Beat heavy cream with a heavy-duty stand mixer, fitted with a whisk attachment, on medium-high speed until stiff peaks form. Gently fold whipped cream into mascarpone mixture.

To assemble trifle: Cut cooled cake into 1-inch cubes. Layer one-third of the cake cubes in bottom of a 4-quart trifle dish. Top with one-third each of peach slices with juice and whipped cream mixture. Repeat layers twice. Serve immediately, or cover and chill up to 24 hours.