First Step: Prepare Oven and Baking Pan Preheat your oven to 350°F (180°C). Lightly grease and flour a 9-inch cake pan or use a springform pan. Line the bottom with parchment paper to prevent sticking and ease cake removal.

Second Step: Mix Dry Ingredients In a medium-sized bowl, whisk together the all-purpose flour and baking powder. Set this aside while you prepare the wet mixture and crumb topping.

Third Step: Cream Butter and Sugar In a large mixing bowl, beat together the softened butter (or olive oil) and sugar until the mixture becomes light, fluffy, and creamy about 2 to 5 minutes. Add vanilla extract, lemon zest if using, and salt; mix well to combine.

Fourth Step: Add Eggs and Dry Ingredients One at a time, beat in the eggs and egg yolk into the butter mixture, beating for 2-3 minutes to ensure proper incorporation. Gradually add the dry ingredients alternated with seltzer water or club soda, mixing gently with a spatula to avoid deflating the batter.

Fifth Step: Fold in Berries Carefully fold the fresh or frozen berries into the batter to distribute evenly. If using frozen berries, toss them lightly in about 1 tablespoon of flour before folding to prevent sinking during baking.

Sixth Step: Prepare Crumb Topping In a small bowl, combine flour, brown or granulated sugar, and a pinch of salt. Cut in the cold, cubed butter using a pastry cutter, two forks, or your fingers until the mixture forms coarse crumbs.

Seventh Step: Assemble and Bake Pour the batter into the prepared pan, smoothing the surface with a spatula. Evenly sprinkle the crumb topping over the surface of the batter. Bake at 350°F (180°C) for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. Check doneness starting at around 45 minutes, especially if using frozen berries.

Final Step: Cool and Serve Allow the cake to cool in the pan for 15 minutes, then transfer to a wire rack to cool completely. Serve at room temperature or slightly warm for a comforting treat. For vegan or gluten-free variations, adjust ingredient substitutions accordingly during mixing and baking.