Macerate the strawberries: In a medium bowl, toss the sliced strawberries with 1/4 cup sugar and 1 tbsp lemon juice. Stir gently to coat. Let sit at room temperature for at least 30 minutes, stirring once or twice, until the berries are soft and syrupy.
Preheat and prepare pan: Preheat oven to 350°F (175°C). Grease your half-sheet pan and line it with parchment paper, leaving a bit of overhang on the long sides for easy lifting.
Whisk dry ingredients: In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside.
Cream butter and sugar: In a stand mixer fitted with the paddle attachment (or using a hand mixer), beat the softened butter and 1 1/2 cups granulated sugar on medium-high until light and fluffy, about 3 to 4 minutes. Scrape down the sides of the bowl.
Add eggs and vanilla: Beat in the eggs one at a time, then add the vanilla extract. The mixture may look slightly curdled — that is okay.
Alternate dry and buttermilk: With the mixer on low, add the flour mixture in three additions, alternating with the buttermilk, starting and ending with the flour. Mix until just combined. A few small lumps are fine — do not overmix.
Spread and bake: Pour the batter into the prepared pan and spread it evenly to the edges using an offset spatula. Bake for 20 to 22 minutes, until the edges are golden and the center springs back when gently pressed. A toothpick inserted in the center should come out with a few moist crumbs, not wet batter.
Cool completely: Let the cake cool in the pan on a wire rack for about 15 minutes, then use the parchment overhang to lift it out onto the rack to cool completely. Do not skip this step — a warm cake will melt the whipped cream.
Whip the cream: In a cold bowl, beat the heavy cream and powdered sugar on medium-high until soft peaks form. Be careful not to over-whip.
Assemble and serve: Spread the whipped cream evenly over the cooled cake. Using a slotted spoon, scatter the macerated strawberries over the top, then drizzle a few spoonfuls of the strawberry syrup over the berries. Cut into squares and serve within 2 hours of assembling.