Line an 8 × 4-inch (20 × 10 cm) pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a medium bowl, combine the oat flour, powdered milk, and a pinch of salt. Stir well to evenly distribute everything.
Add the lemon zest and fresh lemon juice.
Pour in the melted coconut oil and honey.
Stir until the mixture comes together into a soft, slightly sticky dough. It should hold its shape when pressed between your fingers. If it feels too dry, add 1–2 teaspoons milk or water. If too soft, add a little more oat flour.
Transfer the mixture to the prepared pan.
Press it down firmly and evenly, using the back of a spoon or the bottom of a glass.
In a small bowl, prepare the topping by mixing the powdered milk, melted coconut oil, honey, and lemon juice.
Mix until smooth and creamy.
Spread the topping evenly over the base, smoothing the surface with a spoon or spatula.
Finish with a little extra lemon zest on top.
Refrigerate for at least 30–60 minutes, or until the bars are firm to the touch.
Lift the bars out using the parchment paper.
Slice into portions.
Store in the refrigerator, or freeze for longer storage.