Make the streusel first. Heat a conventional oven to 350°F (175°C). Grease an 8-inch square metal pan and line the base with parchment. Mix the streusel flour, brown sugar, cinnamon, and pinch of salt, then stir in its 57 g melted cooled butter. Squeeze gently into irregular crumbs and set aside while preparing the cake.

Mix the dry cake ingredients. In a separate bowl, whisk the cake flour, baking powder, baking soda, and salt. These amounts are for the cake only; do not include the streusel flour again. Stir the yogurt and milk together in a small jug or bowl so they can be added evenly without prolonged beating later.

Cream butter and sugar. Beat the softened 85 g cake butter with granulated sugar for about 2–3 minutes until lighter and evenly combined, scraping the bowl. Beat in the egg and vanilla until incorporated. The mixture may look slightly textured, but there should not be large cold pieces of butter that remain unmixed.

Finish the batter and top. On low speed or with a spatula, add half the dry cake mixture, then the yogurt-milk mixture, followed by the remaining dry ingredients. Mix just until incorporated. Spread in the pan, reaching the corners. Scatter the streusel across the surface in an even layer, retaining small and medium crumbs rather than pressing it into a solid lid.

Bake the middle through. Bake about 42–50 minutes. A tester inserted deep into the cake center should come out without wet batter; a few dry topping crumbs are expected. The center should spring back gently under a light touch. If the topping browns too quickly, lay foil loosely over the pan and continue until the cake underneath is baked.

Cool before cutting. Let the cake cool on a rack in its pan for at least 30 minutes before cutting into nine squares. The topping is fragile while hot and becomes easier to handle as it cools. For wrapping or freezing, wait until fully cool so moisture does not collect under the streusel.