Lemon Buttermilk Sheet Cake with Strawberry-Raspberry Filling

Steps

1. Make the filling first: in a small saucepan, stir the strawberries, raspberries, one sixth of the granulated sugar (about 50 g), the cornstarch and one third of the lemon juice (about 15 g). Simmer over medium heat, stirring, 5 to 7 minutes until thick and glossy. Spread on a plate and let cool about 20 minutes.
2. While the filling cools, preheat the oven to 350°F (175°C) and lightly butter a 9x13-inch pan; line the bottom with parchment.
3. In a bowl, whisk the cake flour, baking powder, baking soda and salt.
4. In a large bowl, rub the lemon zest into the remaining five sixths of the granulated sugar (about 250 g) with your fingers until fragrant. Add the butter and beat 3 to 4 minutes until pale and fluffy.
5. Beat in the eggs one at a time, then the vanilla.
6. Add the flour mixture in three parts, alternating with the buttermilk in two parts, mixing on low just until combined.
7. Spread half of the batter in the pan. Dot the cooled berry filling over it, leaving a 1-inch border, then spoon the remaining half of the batter on top and gently spread to cover.
8. Bake 35 to 40 minutes, until golden and a toothpick in the cake (not the filling) comes out clean.
9. Cool the cake in the pan on a rack for at least 1 hour, until fully cool.
10. For the glaze, whisk the powdered sugar with the remaining two thirds of the lemon juice (about 30 g) until smooth and pourable.
11. Pour the glaze over the cooled cake, spread it to the edges and let it set 15 minutes. Cut into 12 slices.