Preheat oven to 350ºF. Line an 8x8" square baking pan with parchment paper, or grease well with butter and dust with flour.
Combine butter and sugars in bowl of standing mixer and beat until fluffy and pale, about 2 minutes, scraping down sides occasionally. Beat in egg and vanilla.
In a separate bowl, whisk together flour, baking soda and spices. With mixer speed on low, gradually incorporate about 1/3 of the flour mixture, followed by 1/2 of the buttermilk. Repeat, ending with final third of flour.
Spread batter evenly into prepared pan, then cover the surface with apple slices. In a small bowl, combine topping ingredients with a spoon, then use your fingers to pinch off small pieces to drop on top of the apples. Bake for about 50 minutes, or until top is caramelized and centre feels slightly springy when gently pressed.