How to:
Dough:
The dough needs to be made the day or night before you assemble and bake your gata. Start by adding the baking soda and baking powder to the sour cream and mix it until thoroughly combined. Set aside.
Add the flour and the butter to the bowl of a food processor. Pulse the mixture several times until it looks sandy.
Add the sour cream mixture and mix until it comes together into a doughy mass. Remove the dough to a lightly floured work surface. (Do not knead it, but make sure it’s all incorporated.).
Cut the dough into two equal parts. One at a time, on a lightly floured surface, use your hands to press each dough piece flat, about 1cm thick, then fold it into thirds like a letter, turn 45 degrees and fold again into thirds to yield a perfect square. Use your hands to straighten the edges and make sure the corners are relatively sharp. Wrap in plastic and place in an air-tight container and refrigerate overnight.
Filling :
The next day, Add the flour and butter to the bowl of a food processor and pulse the mixture several times until it looks sandy. Add the sugar and vanilla and quickly pulse just a few more times to combine, but not enough to form a dough. Pour this crumbly filling into a wide, flat bowl and use a large spoon to divide it into two. Set aside.
Assembly:
Preheat the oven to 175°C. Line one baking sheet with parchment paper and set aside.
Remove the cold dough squares from the refrigerator and put them on a lightly floured work surface. Gently roll out one of the dough squares into a rectangle about 2mm thick (about 30cm* 38cm rectangle). Do your best to create straight edges and corners for your rectangle, and an even thickness, dusting with minimal flour. Arrange the rectangle so the longest edge is facing you with the shorter edges on either side.
Add half of the filling to the rectangle and use your hands to spread it over the surface, leaving about a 2 cm border at the furthest edge clear of filling, but otherwise spreading the filling from edge to edge. Use the palms of your hands to firmly press the filling into the dough. (This will make it easier to roll without the filling falling out too much).
Beginning with the edge closest to you, start rolling the dough tightly. When finished rolling, gently press the top of the roll to help seal.
Carefully move your roll to a clean piece of parchment or wax paper on your work surface. Use your rolling pin to gently flatten the roll starting at the center and moving outward, and then again starting at the center and moving outward in the opposite direction. You are not rolling it out, but just using the natural momentum of the rolling pin to flatten it.
Brush the top and sides of the roll with the beaten egg. Cut the dough into 3cm slices. Very gently use a small spatula or your dough scraper to remove each piece to the prepared pans.
Repeat the process with the remaining dough square.
Finishing:
Bake the gata on the middle rack for about 25 to 35 minutes, until the tops are dark golden brown, and the dough on the sides is no longer pasty looking, and appears to be cooked through (it may even start to look slightly golden).
Cool the gata completely before removing from the baking sheets. To store, place them either on a large baking sheet, pan, or serving dish, but cover with a large tea towel as opposed to foil or plastic wrap, which will cause the gata to soften.