9 inch spring form pan (or other 9 inch round or square baking pan)
1/3cupall purpose flour
4Tbspcold unsalted butter, cut in small pieces
1/2cupunsalted butter, softened (1 stick)
1largeegg, room temperature
2 1/2tspbaking powder
2cupsall purpose flour
2cupsfrozen raspberries (don't thaw)
Preheat oven to 375F Grease a 9 inch spring form pan. I like to wrap the bottom half of the pan tightly in foil to prevent any leaks. Place the pan on a baking sheet.
Mix the streusel together in a small bowl, using your fingertips to work the butter in to the flour and sugar until the butter is evenly dispersed and the mixture is crumbly. You can also do this in your food processor, using the pulse button. Set aside.
Cream the butter and sugar together for a couple of minutes until light and fluffy either in a stand mixer or by hand in a large bowl. Scrape down the sides of the bowl as necessary.
Beat in the egg until completely incorporated, scraping down the bowl again. Then beat in the buttermilk and extract. The mixture may look curdled, that's fine.
Blend in the baking powder and salt, then fold in the flour. Finally fold in the frozen raspberries, stirring just enough to get them evenly distributed. Turn the batter into your prepared pan and smooth out evenly. Sprinkle the streusel over the top.
Bake for about 55-60 minutes or until just beginning to turn golden and a toothpick inserted in the center comes out without wet batter clinging to it (moist crumbs are fine.) Cover loosely with foil if it seems to be browning too quickly.
Cool 10 minutes in the pan, then gently loosen and remove the sides. The cake can be enjoyed warm or at room temperature.
You can use fresh raspberries but frozen work particularly well because they don't fall apart when you fold them into the cake batter. When I find seasonal raspberries I buy them in bulk and freeze them for later.