How to Make It
Preheat the oven to 350° and butter a 9-inch square metal baking pan. In a medium bowl, mix the brown sugar with the flour, cinnamon and salt. Add the 4 tablespoons of diced butter and, using your fingers, pinch it into the dry ingredients until evenly moistened, then press the mixture into clumps. Refrigerate the streusel until chilled, about 15 minutes.
In a medium bowl, whisk the flour with the baking powder, cinnamon and salt. In a large bowl, using an electric mixer, beat the butter with the granulated sugar at medium speed until fluffy, 2 minutes. Beat in the eggs 1 at a time. Scrape down the side of the bowl, then beat in the dry ingredients and milk in 3 alternating batches, starting and ending with the dry ingredients, until just incorporated.
Scrape the batter into the prepared pan, spreading it in an even layer. Dollop the pear preserves evenly in the batter and sprinkle the streusel evenly on top. Bake for about 50 minutes, until a toothpick inserted in the center of the cake comes out clean. (Some of the streusel will sink into the cake.) Transfer the pan to a rack and let the cake cool completely, about 1 hour. Dust with confectioners’ sugar, cut into squares and serve.