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1
Combine graham cracker crumbs, 1/4 cup sugar, cocoa powder, and melted butter in a bowl until mixture is thoroughly combined and crumbly.
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2
Transfer crust mixture to a 9x11-inch baking dish.
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3
Press crust into bottom of dish until smooth and even.
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4
Cover dish with plastic wrap and refrigerate until set, about 30 minutes.
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5
Mix cream cheese and mascarpone cheese in a bowl until thoroughly combined.
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6
Stir in lemon zest, lemon juice, and vanilla extract into mixture.
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7
Whisk 1/3 cup sugar into whipping cream in a separate metal or glass bowl until the cream is fluffy and forms soft peaks.
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8
Add whipped cream to the cream cheese mixture and gently whisk until filling is soft, fluffy, and well combined.
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9
Spoon filling on top of the graham cracker crust, spreading and smoothing the top.
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10
Tap the pan gently on a work surface several times to settle the crust and filling.
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11
Cover pan tightly with plastic wrap and refrigerate until chilled and set, at least 3 hours.
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12
Starting at the bottom long edge of the cake, arrange strawberry halves in a horizontal line with the pointed bottoms of the strawberry halves pointing to the right.
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13
Start a second stripe of strawberry halves at the top right of the cake, arranging strawberries with the bottoms pointing right in a line stretching about 2/3 of the length of the cake.
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14
Continue to make strawberry stripes, starting about 3/4 inch up from the bottom stripe and laying down 3 more stripes stretching the full length of the cake.
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15
Make 1 more line of strawberries beneath the short stripe to leave a square space for the blue field of stars.
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16
Lay a line of blueberries with the blossom ends pointing up in the upper left square space.
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17
Continue to lay down blueberries in lines, berries touching the previous line, until the square is filled with blueberries.
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18
Cut cake into squares to serve.
