Preheat oven to 350 F.

Line a 9x9-inch or 9x13 inch pan with parchment paper. Set aside.

In a bowl, combine the flour, salt and baking powder.

Beat together the butter and sugar with a mixer for 3 minutes, scrape down the sides and bottom of the bowl. Add eggs and vanilla and beat until smooth (for 1-2 minutes).

Add in half of the dry ingredients, then mix to combine. Next add the milk mixed with lemon juice and beat until incorporated. Add the rest of the dry ingredients and beat until just combined.

In a bowl combine the ingredients for the filling. We need softened butter for it. Press with your fingers to form soft crumbs.

For the Streusel topping:

Make sure the butter is cold. Place ingredients in a food processor and pulse 5-6 times, until crumbs are formed.

Spread 1/2 of the cake batter on the bottom of the baking dish. Add the filling in a n even layer on top. Spread the remaining batter. Top with the Streussel. The topping might seem a little too much, so keep in mind that you can leave some out.

Bake for 45 minutes (between 40 and 50, depending on the oven), until toothpick inserted comes out clean. Let the cake cool to room temperature and cut into squares.