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1
Preheat oven to 350 degrees.
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2
Grease an 8-inch square baking pan and dust it with flour, knocking out an extra.
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3
Combine the eggs, egg yolk, sour cream, lemon zest, and vanilla in a large glass measuring cup and lightly beat.
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4
Combine 1 1/4 cups flour, the baking powder, baking soda, and salt in a medium mixing bowl.
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5
Combine the butter and sugar in a large mixing bowl and cream with an electric mixer on med-high speed until fluffy, about 3 minutes, scraping down the sides of the bowl once or twice as necessary.
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6
With the mixer on med-low speed, pour the egg mixture into the bowl in a slow stream, stopping the mixer once or twice to scrape down the bowl.
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7
Turn the mixer on low speed and add the flour mixture, 1/2 cup at a time, scraping down the sides of the bowl after each addition.
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8
After the last addition, mix for 30 seconds on medium speed.
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9
Combine the strawberries and remaining 1/4 cup of flour in a medium bowl and toss to coat.
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10
Fold the flour-covered berries along with the chocolate chips into the batter with a rubber spatula.
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11
Scrape the batter into the prepared pan and smooth the top with a spatula.
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12
Bake the cake until it is golden and a toothpick inserted in the center comes out clean, 45-50 minutes.
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13
Let the cake cool in the pan for 5 minutes, invert it onto a wire rack, and then turn it right side up on a rack to cool completely.
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14
Cut into 9 squares and serve.
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15
Store uneaten squares in a cake keeper or wrap in plastic and store at room temperature for up to 3 days.
