Place a rack in middle of oven; preheat to 350°. Lightly coat an 8x8" baking dish or pan with unsalted butter and line with 2 overlapping pieces of parchment paper, leaving some overhang on long sides. Using your hands, mix together ¾ cup (94 g) all-purpose flour, ⅔ cup (76 g) coarsely chopped raw pecans, ½ cup (packed; 100 g) light brown sugar, 4 Tbsp. unsalted butter, melted, slightly cooled, 1 tsp. ground cinnamon, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a medium bowl, squeezing gently to create moist, irregular-size clumps; set streusel aside.

Melt 6 Tbsp. unsalted butter in a medium saucepan over medium heat. Continue to cook, stirring often with a heatproof rubber spatula to prevent burning, until butter foams, then browns, 5–8 minutes. Immediately pour into a medium heatproof bowl, scraping in any browned bits stuck to bottom of pan. Let cool slightly.

Vigorously whisk 2 large eggs, 1¼ cups (250 g) granulated sugar, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a large bowl until smooth. Add brown butter, 1 cup pumpkin purée, ¾ cup well-stirred unsweetened coconut milk, room temperature, and 1 Tbsp. vanilla extract and whisk until smooth. Add 1⅔ cups (254 g) mochiko (sweet rice flour), 1½ tsp. baking powder, 1½ tsp. ground cinnamon, ½ tsp. ground allspice, and ½ tsp. ground ginger and vigorously whisk until smooth (batter will be thick). Scrape into prepared pan and scatter reserved streusel over in an even layer.

Bake cake until edges start to pull away from sides of pan, top is deep golden brown, and a tester inserted into the center comes out clean, 60–65 minutes if baking in a metal pan, 65–70 minutes for glass. Transfer pan to a wire rack and let cake cool in pan 15 minutes. Turn cake out onto rack; peel away parchment and discard. Carefully turn cake right side up and let cool completely before slicing, at least 2 hours. Do Ahead: Cake can be baked 1 day ahead. Store loosely covered at room temperature.