Soft Butter Cake

In a small bowl, using a fork, mix ¾ cup of the flour with the baking powder, stirring for 1 minute to aerate. Add to the butter mixture and mix on low speed just to incorporate. Do not overmix.
Add half of the milk and mix for a few seconds to combine. Add the remaining milk, mix again to combine, then mix in the remaining flour.
Add half of the egg whites and mix on low speed to combine. Using a spatula, fold the remaining egg whites into the batter. Be patient—it may take quite a few turns of the bowl to fold in the egg whites. My trick is to turn the bowl as I fold. What I do is to do a “quarter turn” of the bowl (as if I’m turning the bowl from a 0-minute position to a 15-minute position) and fold; turn the bowl to a 30-minute position and fold; turn the bowl to a 45-minute position and fold; turn the bowl to a 0-minute position and fold. Repeat the turning of the bowl and folding until all egg whites are well incorporated.
Pour the batter into the prepared pan and level with a spatula. Bake for 45 minutes. Reduce the oven temperature to 300°F and continue to bake for about 9 minutes, until a tester inserted into the center of the cake comes out clean. You will see that the cake has risen to the very top of the pan but the surface remains flat (don’t worry if you see a few bubbles).
Let cool on a wire rack for 15 to 30 minutes, then invert on a plate, then invert again onto the rack or a platter. I like to wait until the day after to eat it, but you don’t have to. Let cool for a couple of hours before serving.