This raspberry sheet cake is equal parts delicious and easy to make. With a rich, melt-in-your-mouth sponge dotted with raspberries and a pretty raspberry swirled mascarpone frosting, it’s a joy to make and eat.
Pre-heat the oven to 355 ºF (180 ºC) and line a 9 x 13 inch (23 x 33 cm) rectangular sheet pan with baking/greaseproof paper.
In a large bowl, sift together the gluten free flour blend, almond flour, xanthan gum and baking powder. Add the sugar and salt, and mix until combined.
Add the softened butter and, using either a stand mixer with the paddle attachment or a hand mixer with the double beater attachments, work the butter into the dry ingredients until you get a texture resembling coarse sand.
Add the lemon zest. In a separate bowl, mix together the eggs, milk, vanilla and lemon juice. Add the wet ingredients to the dry+butter mixture, and whisk/mix well until you get a smooth batter with no flour clumps.
Transfer the cake batter into the prepared sheet pan. I recommend doing this in several layers, sprinkling each layer with the frozen raspberries to get an even distribution of fruit in each slice.
Smooth out the top, sprinkle with any remaining frozen raspberries, and bake in the pre-heated oven at 355 ºF (180 ºC) for about 38 - 40 minutes or until risen, golden brown on top and an inserted toothpick comes out clean.
If the cake starts browning too quickly or too much, cover it with aluminium foil and continue baking until done.
Allow to cool in the sheet pan for about 10 minutes, then remove from the pan, and allow to cool completely on a cooling rack. (Alternatively, you can leave it in the sheet pan to cool completely – this will depend on how you want to serve the sheet cake.)
First, make the raspberry reduction:
In a saucepan, cook the raspberries on medium-high heat until they have released their juice and part of the liquid has evaporated.
Pass the raspberry mixture through a sieve to remove the seeds and skin.
Return the raspberry juice to the saucepan and cook further until it has decreased in volume to about 1/4 - 1/3 cup.
Allow to cool.
In a big bowl, whip together heavy/double cream and powdered sugar until soft peaks form. You can use a stand or hand mixer – or even do it by hand.
Once at the soft peak stage, fold in the mascarpone cheese. You will get a smooth frosting that spreads easily but will also hold its shape. Add in the vanilla paste (optional) and mix until it's evenly distributed.
Spread the frosting on top of the cooled raspberry sheet cake, and swirl through the cooled raspberry reduction.
The raspberry sheet cake keeps well covered in cling film (or in a closed container) in a cool dry place for 3 - 4 days.