• Preheat oven to 375F Grease a 9 inch spring form pan.
• Mix the streusel together in a bowl, using your fingertips to work the butter in to the flour, sugar, and pecans until the butter is evenly dispersed and the mixture is crumbly. Take the time to make sure this is thoroughly combined. Set aside.
• Start the cake batter: cream the butter and sugar together for a few 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, vanilla, and spices. The mixture may look slightly curdled, that's fine.
• Blend in the baking powder and salt, then fold in the flour. Finally fold in the diced pears, stirring just enough to get them evenly distributed. Turn the batter into your prepared pan and smooth out evenly. Sprinkle the streusel evenly over the top, making sure to get into the edges, too.
• Bake for about 45-50 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.) My cake took 48 minutes.
• Cool 10 minutes in the pan, then gently loosen and remove the sides. The cake can be enjoyed warm or at room temperature.

to make the optional custard sauce

• Bring the milk or half and half just to a simmer over medium heat. Meanwhile whisk the yolks and sugar until well combined. Drizzle a little of the hot milk into the egg mixture, whisking all the time. Drizzle a little more, then transfer that back into the pan of hot milk and continue cooking, stirring constantly, until the mixture coats the back of a spoon. Do this slowly, over medium heat, and I like to use a silicone spoon or spatula to scrape the bottom and sides of the pan as it heats. The mixture will become velvety and thickend, but it will not be as thick as pudding. Don't overcook or let it boil because it will curdle. Remove from the heat and stir in the vanilla. 
• Pour the custard through a sieve (to catch any stray lumps) into a heat proof jar or bowl and place a piece of plastic wrap onto the surface so it won't form a skin as it cools. It can be used warm or chilled. Store leftovers in the refrigerator.