Prep the pan and oven. Heat the oven to 350°F (175°C). Line the baking pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.
Make the crust. In a bowl, cream the butter, sugar, and salt until smooth and slightly fluffy. Add the flour and mix just until a soft dough forms.
Press and bake. Press the dough evenly into the lined pan. Prick a few times with a fork. Bake for 18–20 minutes, until the edges are lightly golden.
Whisk the dry filling ingredients. In a clean bowl, whisk sugar, flour, and matcha. For a lump-free mixture, sift the matcha first. Stir in the salt.
Add eggs and lemon. Whisk in the eggs one at a time until smooth. Add lemon juice and zest. The batter should be fluid and evenly green.
Pour and bake again. Reduce oven to 325°F (165°C). Pour the filling over the hot crust. Bake 18–22 minutes, until the center is just set and no longer jiggly at the surface.
Cool completely. Let the pan cool on a rack for 1 hour, then chill at least 2 hours. This helps the custard fully set and slice cleanly.
Decorate. Lift the slab out using the parchment. Dust lightly with powdered sugar if using. Arrange edible flowers on top—press gently so they adhere.
Slice and serve. Use a sharp knife, wiping between cuts. Cut into 16–24 bars, depending on the size you prefer.